You're a House Lord, with lots of factories.
Yay!
You can build any single design, but only one design of each general weight class;
Awww.
I do presume you mean that, between those 4 weight classes, hundreds will be made in some kind of distribution I control? IE 4000 Mechs in a 10:20:60:10 distribution as a clumsy example? Volume matters.
Mech forces:
These will do most of the fighting, leaving non-fighting tasks to vehicles. Inability to change variants means the only modularity is ammo types, so not even going to try to lift those jobs with Mechs.
Light:
Spector SPR-5S. JJs, Stealth Armor and high mobility means that companies of these guys can get where I want them and without the enemy being able to do a whole lot about it. Some range, so these guys can do what Light combat units are supposed to do: pick your target, and have the speed to avoid fights you don't want.
Their job is *NOT* recon, but they wouldn't completely suck at it.
Medium
Starslayer 3D. Job: Cavalry. They're quick enough to fill (or exploit) strategic holes as they appear, and durable enough (no XL, full armor, no ammo) to hang for a while until the Heavies get there. Range of weapons means that there's not a lot of weaponry out there that can shell Starslayer companies without getting some PPC for their trouble.
They lack the staying power to be a main line unit, and wouldn't be used that way. (See also my above distribution of units)
Heavy
Dragonfire DGR-4F. Main Battle. These guys are how battles are won, the other Mechs just provide strategic utility in a ground war. Romping in battalion-size formations (or larger) their main weapons wreck whatever they can see.
LBX provides some AA utility against VTOLs until they can be chased off by aerospace. Also good at neutralizing vehicles.
Pulse lasers give an answer against fast units trying to skirmish. ECM gives options against C3 enemies.
GR permits some parity with Clans.
XL a bit of a liability, but these guys should withdraw when their ammo runs low anyway, and shouldn't keep fighting when the armor gives.
Standard mode, these formations would avoid physical combat (either by never closing with the enemy, or by killing whatever tries to approach), so lack of heat weaponry to neutralize TSM should in practice not become a huge problem. If the Capellans bring a battalion of Ti Tsangs, well, things are just going to suck a little for a while.
Presuming the optional rule of shutting down Gauss Rifles, these guys shouldn't be too prone to ammo/XL related catastrophies.
Assault
Imp IMP-1B. Role is to hold what must be held, or to break what's not mobile. Deployed in company strength or above, these are marched in to stationary targets we can't degrade sufficiently with artillery or aero. Or they're deployed (via DropShip) to hold positions we can't lose, reinforcing the Starslayers in a gap, and holding until the Dragonfires show to take care of the problem.
Zombie with pretty solid range capabilities, LRM20 is there to provide some all-round utility. IE Frag to clear woods and infantry, incendiary and smoke ammo to bring your own cover, or to provide some Indirect capability. Sop would be to dump ammo, rather than compromise the unit's zombie capability; depending on known opposition, they might deploy with minimal ammo just to enhance their survival rate.
You want vehicles too?
Light and medium: some kind of setup where they do 99% of recon and infantry mobility. IE, a Maxim and a light vehicle for scouting, or a light vehicle for the infantry, and a medium for scouting.
Heavy: LRM carrier.
Assault: Some semi-mobile pillbox
VTOL gunship: Whatever has LRMs or better and a lot of speed.
VTOL cargo carrier: toss-up between going heavy cargo and slow speed, or high speed, low cargo, but zillions of them. IE Karnov vs. Ferret
Aero:
Light: Knifefighting dogfight bastards.
Medium: Aerospace superiority
Heavy: DropShip assaulters/bombers
Artillery:
Plenty, though all Arrow IV if I can find a Light/Medium with TAG, since I couldn't park any on the Mechs.
Artillery rules suuuuuck.
Paul