to the original question: a LOT of the correct answer has to do with how you set up your doctrine.
1. is the force intended to be mobile/easily deployable offworld?
2. what does your other supporting arms look like?
3. how much logistics are you going to handwave?
Answer 1 has to do with the size of vehicle bays on dropships. Under 50 tons for the most advantageous bays for a force that is expected to go places OTHER than their home planet.
Now, since I currently don't have any generation software installed on this computer...
My answer to (1) was "We don't want to leave home or go very far." So, I went heavier than is optimal for a unit with interstellar deployment ideas.
I took the base model Rommel/Pattons from TRO 3025 as my basis.
Three basic chassis that share lots of components (drivetrain, armor, etc.)
The base model, is your MBT, which is a Patton with the AC/10 swapped out for an LBX-10, the flamer and laser swapped for an AMS and half a ton of ammo, and some armor loss to install a coaxial small pulse laser (since we've got those 10 heat sinks).
The Infantry carrier loses the main gun for a standard PPC and a four ton infantry bay crammed into the hull, the engine and final drive being relocated to the front of the tank to give it a 'drop door' in the rear, everything is kept at the same tonnage and the only weapon in common with the MBT is the LRM 5, which is loaded with smoke or Thunder.
The Mobile artillery is a Rommel with the main gun swapped for a Thumper with the same ammo complement. this is followed by a prime mover using the same chassis minus turret, that has a lift hoist and lots of ammunition storage.
Supplementary vehicles using the same engine and chassis include a CEV with a bulldozer blade, backhoe, towing adapter and machine guns.
The Command post model reverts to the PPC and five tons of Commo gear.
using "Efficiencies of scale" (a dirty word in the Battletech setting) this makes maximum use of every ton of stored parts *(or max use of every pallet of transported spares provided you can wrangle enough dropship lift to move the unit).
A nice person took my basic ideas, and did a wheeled amphib 'scout vehicle' that also uses the same engine, we're calling it the 'Fennec Marine Scout car".
Why use one common chassis for all this? simply logistics. It's more efficient if your spare parts fit without a lot of custom hand-fitting. In realistic settings your mechanics will have a much easier training cycle if they only need to learn one engine, one gearbox, and so on, and your drivers can be trained much faster if they only have to learn one instrument panel and control layout.
but this can be done with nearly ANY type of vehicle within the 45 to 70 ton range, with the possible exception of VTOLs or submarines.
My "Ideal" therefore, has a LOT to do with the idea that armies need efficient supply chains that make the most use of each kilo or cubic centimeter of storage and transport.
Typically in CANON battletech, a battalion of armor is 36 vehicles...and often 37 different types in the same battalion.
Simplified logistics (common fusion power plant, common drivetrain and gear box components, only variations where you HAVE to have them, common armor composite type and so on...) on the strategic level means forces have more endurance in the field, and easier training cycles on the support end. (If your guns don't go 'bang' then it doesn't matter that they're bigger, if your tanks are hobbled by the wrong spares, they don't do so hot on the field problem, and so on...)
I chose the LBX because it's got two things going for it:
A) it's stone-ass reliable. With a shell in the chamber, when the gunner presses the firing button, it goes 'bang'.
B) it's a very Versatile weapon that can be used to clear enemy infantry, knock out enemy air support, and still engage hard targets like other tanks or even 'mechs reliably.
I chose AMS because of game-considerations (crit seekers are most of the time missiles, and Infernoes are still deadlier to conventional forces than they are to 'mechs).
The pulse-laser is more of a gimme to the tech-heads out there, my own preference would be a medium laser for heavy work and maybe sacrifice some protection for machine-guns to deal with crunchies.
The choice of the Thumper has to do with a doctrine of fire-and-move where mobility is actually important, as is the ability to SUSTAIN FIRE in the role of terrain denial or screening an advance. It doesn't do the damage a Sniper or Long Tom does, but it's got a DEEP ammo bay, and with firing units of three tubes to an FDC (or four) you can lay down patterns of fire ahead of your infantry advance.
The layout of the IFV version, is plotted specifically with the idea that the IFV is supporting the crunchies on the ground directly after they dismount, mainly in the role of lighting up anything too tough for infantry weapons to chew through.
My typical Kowloonese Battalion would be one company of MBT, two of IFV's, and a firing battery of artillery with a scout section attached at battalion level. 36 vehicles, 4 types, with two companies of infantry and a Battlesuit platoon.
Scaling up to Regimental, and you add an engineering company with the CEV version, additional CP and FDC vehicles, and an aviation section running common (cheap) spotter VTOLs.
The key factors here being that it's easy on the inventory, and training doesn't have to be as broad-based for maintenance-your crews can handle most light maintenance tasks, while battalion, regimental and depot levels don't have to keep a broad base of books to handle the wide variety of vehicles most House and Merc units are saddled with.
It's expensive to buy, but cheap to maintain, diggit?
