I am trying to figure out a "real life" set of armies for 3025 that includes battle armor.
Depends on what you mean with '3025'.
If you mean, the factions that existed in 3025, then only the Clans would have Battle Armor, on their home worlds.
ComStar also has some (PAL) but they don't deploy it openly in 3025.
If you mean the technology in 3025, so effectively introductory, then BA is technically not a part of 3025 tech. But yes, if that's the only change, then using APCs is definitely the way to go.
So, leg infantry, mechanized infantry (probably in 20 ton APC's so you can deploy a platoon), battle armor, vehicle units of several varieties, mechs, artillery and air support.
Yes, most larger BattleMech formations organize that way, not just RCTs.
If I was defending or invading what percentage of these units would I have? Because of rules or preference I see almost no artillery, little infantry, occasional battle armor, some vehicles and a lot of mechs. Air support is only when the group prefers it.
Around 3025, the majority of Mech regiments organized with 1:1:1. 1 Regiment of Mechs was supported by 1 Regiment of Vehicles (usually combat focussed) and 1 regiment of infantry (not clear if they had their own APCs, but this seems probable)
RCTs are an exception I'm going to ignore.
If you take 1:1:1, and you further decide only 1 company in 10 is equipped with Battle Armor, then that's 3 platoons of 4 squads each, and each squad has 4 suits. So 48 suits for the regiment.
If I was invading planet A with a garrison of about 1 division, what would that division be? Obviously a planet with some terrain could have a leg infantry regiment, a combined mechanized infantry and armor regiment, a mech battalion, 1 armor battalion, artillery and air support, with or without battle armor support. Invading forces might be more mobile, with less leg infantry but battle armor support. Lots to think about.
Only a handful of planets in the known universe would be protected by a division worth of equipment. Some planets have only a few companies of militia tanks and all the infantry they can eat.
Many worlds never see a 'Mech that isn't a pirate or raider. Unless they hire some mercenaries themselves.
I say Division worth of troops, since only the SLDF organized in Divisions: no one does anymore. (ComStar's Divisions are basically 2 regiments of mixed type troops)
But back then, it would be 3 brigades, of 3 regiments each.