Sorry to be a SphereInner....umm....sinner....blessed be Knightmare most merciful spell checker. {This reminds of that scene from Murder by Death when Cow on wall corrects Peter Sellers}
Before there is an interdiction back to the subject at hand. Would you already have a proliferation of primitive designs thought the Human (space) Sphere? The year of our publication will be 2600 so going by the list below;
Terran Hegemony - Mackie (2439)
Lyran Commonwealth - Mackie (2459)
Federated Suns - BattleAxe (2459)
Draconis Combine - Gladiator (2468)
Free Worlds League - Icarus (2470)
Capellan Confederation - Firebee (2472)
Taurian Concordat - Toro (2484)
Outworlds Alliance - Locust (2549)
Magistracy of Canopus - Wasp (2579)
Rim Worlds Republic - ??
As you can see all the major states of humanity have had the ability to construct BattleMechs for at least 21 years and in case of the Inner Sphere over a century before the start of the Reunification War. That gives you a lot of time to build up your Mech forces. Now at this time aren't lords of the Inner Sphere more powerful because of the lack of effect interstellar communications? Wouldn't they be the primary builders and shakers prior to the founding of the Star League and the Reunification War? House Military development had to have gotten a boost thanks to the League. So there could be a load of primitive tech Mechs out there.
Incidentally it helps the Friekorps story I think that they were a bunch of House Troops Regiments disbanded or disenfranchised by the Star League. Driven out of direct service to their Lords mercenary service could be a fun solution.
But back to topic Size of Noble and Mercenary Forces.....