For sake of clarity, my best guess at your composition (based on stated tonnages, likely to be wrong) is something like this:
Battalion HQ Support Company 4887 CP itself containing:
36 heavy Artillery
12 heavy Anti-Aircraft
1 Battalion HQ Vehicle
36 x VEH+HB/HCAP = 98 tons each, 3,528 tons total
12 x VEH+HAA/HCAP = 98 tons each, 1,176 tons total
1 x
with HQ21 or higher, 183 tons
For RP this is fine, but for combat efficiency you want to have 2x HB or HAA and not waste ~25% of your weapons tonnage on HCAP for a rear echelon formation. This would mean fewer vehicles but more actual fire delivered to the targets.
Each Mechanized Infantry Company 5069 CP containing:
12 Superheavy Ogre APCs (Superheavy anti-vehicle, Heavy autocannon & Heavy crew-served AP)
48 Powered Armor Gatling Gunners
96 Powered Infantry
1 Company Command HQ
12 x SHV + SHAV/HAC/HCAP = 280 tons each, 3,360 tons total
48 x INF + HCAP (armored) = 20 tons each, 960 tons total
96 x INF + PWI (armored) = 6 tons each, 576 tons total
1 x SHV+HQ5/HCAP/HCAP = 173 tons
Again for RP this is fine, but for practical use it is probably too heavy which means expensive and supply-intensive. It might depend on what point you're at in the game, but as a self-described newbie I'm guessing you're not too far into your campaign and the kinds of units that require heavy weaponry - not to mention the heavy weaponry itself - is going to be prohibitively expensive to research so early and few if any NPRs would even consider doing so. You can probably get more fire on target, and a higher overall tonnage kill rate, by using regular old MAV, MAC, CAP, and PW.
Standard VEH mounting for the MAV/MAC is fine, you could put a few thousand RPs into heavy vehicles if you wanted to which would blunt enemy MAV pretty effectively. SHV and UHV are so massive that they're really not tonnage-effective ways to deliver firepower early in the game, they're more useful once better techs and bigger units are being fielded as something you can drop into the middle of a battlefield and write "Local Tank is <censored> Invincible" stories about in the propaganda papers rather than as any kind of main line element.
That said, you've broadly got the unit composition right - AV + AC + CAP covers all your bases, while some will denigrate the AC line of weapons it does excel versus light-armored units that AV overkills but (H)CAP doesn't penetrate well. Just you don't need to be so heavy about it, use lighter weapons and more of them to concentrate fire.
Also, since you're close to the 5,000 ton mark try to get at or just under rather than over unless your OOB is derived from RP considerations, it will make transport logistics a lot simpler. Currently your 4-formation battalion (HQ + 3xCO) will require just over 20,000 tons of transport capacity and the largest transport module holds 5k tons. Of course there's nothing wrong with having a 5,069-ton formation, it will just be a bit of a logistical headache for you.
Three battalions together will form a brigade, under a relatively small Brigade HQ (probably just some extra AA). However many supply trucks will be added to the Brigade HQ as availability and need dictate. I'm not sure I will need larger force structures than this. What say you?
Three-level force structures are fine. The only problem with them is that they don't play neatly with the auto-assign, if you have a fourth-level commander at the top of your hierarchy you may run into problems but you can just assign them manually (it's one guy every once in a blue moon, not a big problem) or design a specialized Corps HQ for commanding really big planetary assaults once you get to that point.
Something I am quite unclear about... I should obviously flag the HQ units and supply trucks with "avoid combat". However I want my artillery and anti-aircraft guns to stay off the front lines. Will that happen simply by virtue of their being higher up the hierarchy or should they too be flagged "avoid combat"? Would this perhaps prevent them from bombarding targets and shooting down aircraft?
The effect of the "Avoid Combat" flag is that a unit will have -75% chance to hit or be hit/targeted in ground combat (and only ground combat). As artillery and AA are ground combat elements you do
not want them to have this flag. HQs, FFD, LOG, and other specialized non-combat elements should have this flag set as they do not fire in combat. STOs are a special case, you do want to set that flag even though they are "combat" elements, because they do not participate in ground combat but rather fire at ships in space. Another special case would be if you have a vehicle-mounted non-combat module alongside a secondary weapon such as CAP; in this case it is up to you but I would usually still mark that element to avoid combat as the non-combat module is usually more important than an extra machine gun or whatever. Once you're building UHVs with a HQ module and three HACs then you may want to reconsider that advice.
Summary:
DO set the Avoid Combat flag for: HQ, LOG, FFD, CON, GEO, XEN, and STO elements. These do not shoot in ground combat and thus want to have the extra evasion chance.
DO NOT set the flag for: PW, CAP, AV, AC, Bombardment, and AA elements. You want these units to shoot at full effectiveness.
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Also whatever Jorgen said is probably good advice too.
