Well, the very first mech is actually a 100-tonner. As for primitive mechs, If you use the Against the Bot > CivilianUnits_PrimMech RAT, you can get a spread of primitive versions of different mechs.
Sarna.net Primitive Battlemechs This category gives all the models that have a primitive version somewhere. Overall, they have access to most of the same things, just in lesser quantities.
A game where you only had access to 2 or 3 damage weapons would be interesting - combat would last a lot longer, barring disproportionate crits happening.
edit:
Neonivek - Understood, and will do. I'd like to get you to Regular levels for Piloting and Gunnery first, and then I'll start putting XP into your Mech Tech skills. Once you get to regular, you can take over maintaining your own Mech.
What I'd really like to do is when we get to A-rank, and can have access to factories is sit down with you and figure out what you'd want from a quad... and then the unit builds one.
In the far future, I'd also like to build my homebrew mech form years ago, the ARE-1S Ares, a 90t jump capable mech with two large lasers, six medium lasers and four SRM-4 packs. I recall that it should be able to jump and fire everything but the large lasers every round without heat problems, but I think it was built on a 3050+ tech base to do so.