First up, I like to model a mostly realistic combined arms setup and I figure my base size will be 10k. Calling that a battalion. Mix and match to get up to 100k and that is a division. I intend to have long range, heavy and light bombardment in every division. I've got a lot of that setup but I'm not sure of the optimal way to setup supports.
This is an... interesting way to do it. While nomenclature is a roleplay decision and entirely up to you, I will note in case you're not aware (though I'm sure you probably are) that a battalion is usually closer to 500 men (give or take a fair bit either way) and there is usually a regimental and/or brigade level of command between battalions and divisional HQs. Your infantry battalion for example would probably come out at close to 1,800 soldiers using typical manning requirement for the heavy weapons, which is more commonly considered a smaller regiment or brigade.
This has no effect on the mechanics, again I am just sharing knowledge in case it is not already known.
Am I going to have to have a bombardment for each and every front line unit or is there some way to share those?
A formation can only support one other formation. This has led many players to do things such as, e.g., using smaller artillery units (e.g., artillery companies to support infantry battalions) so that each front-line formation can be supported. In my view, it is better or at least easier to just leave the artillery formation at the same size as the front-line formations and accept that it will only support one front-line formation at a time. In the big scheme of things, with multiple millions of tons of ground units in major homeworld invasions, it's not worth worrying about in the aggregate, and concentrating fire this way is probably more advantageous for secondary mechanics like breakthroughs.
Do the engineers work just by being in the division HQ?
What you have will work fine, as construction units will work on elements in any formation in this order: (1) their own formation, (2) formations subordinate to their own formation, (3) their parent formation, and (4) any formations subordinate to their parent formation. This allows for potential fine control of which formations are being fortified or you can just set-and-forget so to speak.
105mm Howitzer Company
Transport Size: 10,000 tons
Build Cost: 788.3 BP
81x 105mm Howitzer - 2133
76x Heavy PW Infantry 2103
1x Armor HQ 10k - 2133
You should be aware that non-bombardment combat units contribute nothing, aside from acting as mediocre meatshields, in support or rear-echelon formations. In other words, components like PW, CAP, LAV, LAC, etc. only contribute if their formation is on front-line attack or defense stance. Therefore the PW Infantry you have there don't do anything for you aside from costing money and minerals. You can of course leave them for roleplay but I would remove them if you care about mechanical efficacy.
Bradley Company
Transport Size: 10,000 tons
Build Cost: 949.4 BP
38x Heavy AC-MG 2103
20x Bradley Tow AC - 2133
40x Bradley MG - 2133
20x Bradley HCAP - 2133
5x Bradley AA - 2133
1x Armor HQ 10k - 2133
Roleplay comment: It is rather unusual to see Bradleys (I assume these are M2/M3s?) without infantry since they are after all Infantry Fighting Vehicles. Also, typically we would expect to see 40-50 Bradleys in a (mechanized infantry) battalion, so calling a formation of ~120 Bradleys a company is rather unusual, though of course you may do whatever you want.
Otherwise this looks fine, you don't really
need so many subtly different weapon types in the same formation but there is no rule against spending your research points to do so.
Division HQ
Transport Size: 5,000 tons
Build Cost: 270 BP
70x Logi Truck 2107
70x Heavy PW Infantry 2103
1x Mobile HQ 100k - 2133
4x Heavy MG Infantry 2103
Same comment RE: PW and CAP infantry as above.
Infrantry Battalion
Transport Size: 10,000 tons
Build Cost: 406 BP
1330x Heavy PW Infantry 2103
150x Heavy MG Infantry 2103
50x Heavy LAV Infantry 2103
20x Heavy AA Infantry 2103
5x Heavy FFD Infantry 2103
1x Heavy HQ10 Infantry 2103
Mostly looks good, do be aware that AA are mechanically unnecessary unless you have multiple player races, since the NPRs (including spoilers) never use ground support fighters.
While I would never disparage the spelling and grammar of a strange alien language, in English the spelling "Infantry" with just the last 'r' is usually used.
Infrantry Mortar
Transport Size: 5,000 tons
Build Cost: 203 BP
240x Heavy LB Infantry 2103
35x Heavy PW Infantry 2103
1x Heavy HQ Infantry 5k - 2137
I'm not sure what the intention is for this formation. If you intend to use the LB from the support echelon, the PW infantry are again not useful. If you intend to use LB in a front-line formation I would probably just include them into the main Infantry Battalion (they could replace the AA for instance) rather than having them in a separate formation which is a little bit more vulnerable to heavy losses.