Marski goes for the kill straight off the gate! 
Theoatmeal2, if you want to keep it simple, you want 4 formations for your entire army.
First for front-line attack and this is all vehicles
Second for front-line defence and this is all infantry and static
Third for support echelon and this is all medium bombardment in vehicles (so it takes less space per HP/armour than static) and it supports the first formation.
Fourth for rear echelon and this is your heavy bombardment, heavy AA, construction units, FFD units, big HQs and supply units and it supports either first or second formation depending on whether you're on the strategic defensive or offensive
Then you construct equal amounts of each. Their exact sizes depend on how big your troop transports are. I'd say go for 50,000 tons per formation, to begin with, and then bigger once your shipyards can churn out 200,000-ton commercial ships.
This is the simplest organization that still utilizes game mechanics completely. Everything else is roleplaying.
Also consider a 5th type of formation on top of this which is just HQ formations of various sizes. Instead of just having massive 50k formations you might want to have a layered HQ hierarchy involving smaller formations in order to stack various commander bonuses on to your actual fighting formations. Note that the more layers you add the more bonuses (with diminishing returns) but also much more micromanagement during set up.
Vehicles are much more space efficient when it comes to invasions which is why people will recommend you to use vehicle only attack groups. They are also less affected by lack of fortification since they cant fortify as much as infantry to begin with.
I don't go as insane as modelling every squad down to sergeant level command however I consider myself one of the heavier micro types who goes down to company level working my way up to battalion, regiments, brigades and then divisions. This isn't necessary, you could just have brigades and divisions without any sort of complicated hierarchy.
Exactly this.
A better approach is to go for something like 3x 10,000 ton formations for your Front-line/Mid/Support and then have a larger formation around 15,000 tons which is your logistics, construction and such.
For one, it makes your army much more modular and you can tweak the exact composition of a division to fit the exact deployment need without having to design everything from the ground up. Secondly it makes rolling out improved units a heck of a lot easier as you can simply just outright haul out and replace say your entire heavy armoured vehicle formation with a more effective one without breaking apart the rest of the division and do so for multiple divisions at once using your existing troop transports.
Thirdly, training times.
When C# first rolled out, it probably took me a good couple of months before I stopped making 40-80k ton formations with everything in it. The main push to approach it in a more modular fashion came from watching Space Marine design their own ground troops in one of his series and I realised that I had been overlooking the finer points of the 'Order of Battle' tab and how a series of several smaller formations can then be dragged and dropped onto a larger 'Command' formation to really mix things up.
One thing I find myself doing is not really doing a front line/Mid/Rear approach but rather doing a Anti-Vehicle/Anti-Personnel/Support approach to my formations other than some minor differences in formation composition to be much for focused typically the deployment roles are largely the same except if I need to go on the aggressive side of things my front line attackers are typically always my ultra heavy Anti-Vehicles, with my Anti-Personnel sticking to the Front Defence.... in some situations that does change around to deal with specific needs.
Whilst I tend to avoid going too far down the formation compartmentalisation rabbit hole, I typically find myself doing the following almost every campaign regardless to roleplay as a standard 'Garrison' force that can also be deployed elsewhere for standard engagements:
Garrison Command Formation:
* 15,000 Ton Capacity HQ. Depending on Roleplay quirks either as a Ultra-Heavy Vehicle with crap tons of armour or more typically as a static unit.
* Construction Units. Typically I go for making them the largest vehicles I can support with lots of armour. Mostly because I simply like the idea that my fortification units are themselves heavily protected so they can actually get to work fortifying their location.
* Logistics Units. As they're going to be 'consumed' a whole lot. Just Infantry units but often with HPI Armour. At a absolute minimum they need to provide enough logistics for at least 10 rounds of supporting all formations that will be falling under the 'leadership' of the Garrison Command Formation.
Garrison Anti-Vehicle Formation* 10,000 Ton Capacity HQ. Again depending on Roleplay quirks, either beefy vehicles or static.
* Ultra-Heavy Vehicle Heavy Anti-Vehicle weapon team. These things are partly meant to deal with beefy ground units, but another key role is to essentially pose as a very large brick wall for the enemy forces. There typically aren't many of them in the formation as typically I field around 10 of them per formation, but with them sporting 4x heavy anti-vehicle weapons and even earlier racial armour progression can offer up 180 armour with 360 hitpoints, I really don't need many in a single formation of them to serve as a brick wall during engagements.
* Infantry Heavy Power Armoured Light Anti Vehicle team. Cheap, cheerful and can usually fit around 120 or more whilst keeping to the 10,000 Ton ceiling limit for the entire formation.
* Ultra-Heavy Vehicle Heavy Auto Cannon Weapon Team. Honestly just a couple in the formation sometimes if I fancy some variation.
* Logistic units. Again a minimum logistics amount of 10 rounds but this time just feeding its own formation.
Garrison Anti-Personnel Formation* 10,000 Ton Capacity HQ. Again depending on Roleplay quirks, either beefy vehicles or static.
* Ultra-Heavy Vehicle Heavy Crewed Anti-Personnel Weapons Team. Similar to the AV formation. Around 10 of these guys, meant to fill the same general role except aimed at softer targets.
* Ultra-Heavy Vehicle Heavy Auto Cannon Weapon Team. Same folks as from the Anti-Vehicle formation except typically I have twice as many in the AP formation (So normally around 4-5).
* Infantry Heavy Power Armoured Heavy Crewed Anti-Personnel Weapons Team. Similar numbers as their AV counterpart so approximately 110-120.
* Logistic Units. Same rules apply as the other formations.
Garrison Artillery FormationThis is the formation that changes the most between campaigns, typically depending on the kind of 'personality' I feel my race has. But general rule of thumb here is big. Beefy. Long Lasting. Almost always these guys are assigned to support the Anti-Vehicle Team with a support field role.
*10,000 Ton Capacity HQ. Yep, getting lots of usage from this team.
* Ultra-Heavy Vehicle Long Range Bombardment. Large. Fires a lot of artillery shots per unit. Either just a couple in the team, or a dozen of them. Depends on the race 'personality'.
* Ultra-Heavy Vehicle Heavy Bombardment. Large. Heavy Hitting Barrage Unit. Either just a couple or a dozen. Amount swaps between LRBs.
* Ultra-Heavy Vehicle Construction. Honestly, probably completely pointless but I like the idea of my artillery formation having engineers deployed alongside them that deal with emplacement construction. Usually only a handful of them.
* Logistics Unit. You know them already. But unlike all other use cases, there are enough in the artillery formation to support the artillery formation for a minimum of 15 rounds of combat.
Typical hierarchy break down:
- Garrison Command [Rear Echelon]
-- Garrison Anti-Vehicle Team [Front Line Attack/Front Line Defence]
--- Garrison Artillery [Support]
-- Garrison Anti-Personnel Team [Front Line Defence]
Side Formation:
Survey Team - This formation is interchangeable with the Garrison Command Formation for when I want to move troops to a new colony that also has survey or xeno potential.
* 15,000 capacity HQ used as the same garrison command formation.
* Ultra-Heavy Vehicle Xeno Team. Pack around a half dozen of them per formation. All equipment slots used for Xeno.
* Ultra-Heavy Vehicle Geo Team. Same as Xeno but with Geo equipment.
* Ultra-Heavy Vehicle Construction. Same as with the Garrison Command formation, same number of units too.
* Ultra-Heavy Vehicle [Insert Flavour Weapon here]. Very much a token combat unit, weapon type doesn't matter so much. I just like to think my survey team has some kind of protection provided as standard.
* Infantry Heavy Powered Armour [Insert Flavour Weapon here]. Same again, just more of them per formation.
* Logistics Unit. Don't go for quite as many with the Survey Team, usually just enough logistics to support themselves and whatever 3 formations fall under them for 2 or 3 combat rounds. As each formation has enough to logistics to support themselves for almost a dozen turns, the Survey Team logistics is mostly just to provide some additional padding whilst a proper combat command controlled force can be dispatched.
Typical hierarchy break down:
- Survey Team [Rear Echelon]
-- Garrison Anti-Vehicle Team [Front Line Attack/Front Line Defence]
--- Garrison Artillery [Support]
-- Garrison Anti-Personnel Team [Front Line Defence]
Though as can be seen, in both hierarchy break downs, if desired I could go with any mix of compositions and they'll transport just the same.
Ship wise I tend to go for drop pod capable troop carriers with troop capacity of around 45,000 ton each. This means each ship is capable of deploying a entire group in itself of whatever composition is desired.
Edit: Funny enough, hostile forces just suddenly appeared on my homeworld in my campaign as I was writing this.... not entirely sure how they got there as there has been zero ship detection in my home system.... there was a ruin on my homeworld but that was fully excavated like 50 years ago..... so no idea what happened there.... magically teleporting enemy ground troops I guess.
Those Garrison troops are based on very early game racial tech and really could have been replaced, but oh well. Seem to be doing their job OK for such primitives.
