Finally started to code this up after a week or two without being able to spend time on Aurora.
For C# Aurora, a ground 'unit' will be an individual soldier or vehicle, while a group of units will be a 'formation', although you will be able to name formations as companies, battalions, etc..
There will be no restrictions on formation sizes, so you can create anything you want, but there will limitations on the command ability of headquarters based on total formation size.
In terms of 'replacements', I will allow units to move between formations, so you could combine two formations with casualties or use one formation as a source of replacement units for other formations.
While you will build formations according to a template, once they are built each one will be treated as unique formation. So you could have two 'infantry battalions' with different TOE as a result of casualties, replacements or reinforcements.
There are no restriction on units within a formation so you could have a template for an 'Combined battalion' that includes some infantry units, several mortars, a couple of machine gun units, several anti-air units and a few light tanks. Once built, you could add a couple of heavy tanks from a a different formation.
Each individual unit within a formation will fortify separately, so there is no problem fortifying a formation that includes infantry, static and vehicles.
In terms of combat mechanics, units will fire at units, using the chance to hit, armour penetration and damage mechanics previously described. The combat round will run once per hour if no one is attacking, once every ten minutes if one side is attacking and every five minutes if both sides are attacking. I haven't decided yet how to handle aircraft, but I will probably have them attack less frequently but with higher impact.
To simplify things, I will use a low level wealth charge to maintain units outside of combat. Supply Points (same as used by ships) will be used up during combat rounds, so the more intense the combat (as per previous paragraph), the faster the rate that supplies will be consumed. This means you do have to consider the logistics when launching and maintaining an assault, but there is no overhead in micromanagement terms when units are idle.
I haven't decided yet how to handle the actual consumption of supplies but it will likely be either direct from a planetary stockpile, or by using up units with a 'logistics' component.
Supplies will be used during an occupation where the planet is not yet pacified and I could also add some form of training where supplies are used.
In terms of occupation strength / garrison, I will probably base it on the number of individual units in a formation, rather than their capability. So a pure light infantry formation will be much more effective as a garrison force than a considerably more expensive formation of heavy tanks.