As the OP correctly points out, the 'Mech selection in this game was determined by what PGI had ready in MWO more than by existing BattleTech lore.
While it is correct that the salvage mentality of the Succession Wars means that any 'Mech can ultimately turn up anywhere, many of the 'Mechs used by HBS for their game don't belong into the Aurigan Reach region except under extremely unusual circumstances:
- The Commando was supposedly rare outside of the Lyran Commonwealth (even among mercenaries)
- The Spider was a relatively obscure (and rare) 'Mech, never built in large numbers, hard to find replacement parts for, and the only factory destroyed long ago
- The Panther was supposedly rare outside of the Draconis Combine (even among mercenaries)
- The Raven is an anachronism as it was not deployed prior to the 4th Succession War (and even then only rushed to the front out of sheer desperation while still in prototype stage, with sometimes desastrous results like neurofeedback from the ECM killing the MechWarrior)
- The Kintaro was considered extinct
- The Catapult was a relatively obscure (and rare) 'Mech, never built in large numbers
- The Cataphract was a brand-new design from the Capellan Confederation that had just entered production (and was therefore rare to begin with, and unavailable outside of the Confederation)
- The Black Knight was considered extinct
- The Highlander was considered essentially extinct (The Capellan Confederation, while unable to build them, apparently had a small number that they managed to keep operational)
- The King Crab was considered essentially extinct (A handful remained operational in various places, but the design was infamous for being a "hangar queen")
As for which 'Mechs are missing that you would expect to be found in this area:
- The Wasp and Stinger are the other two parts of the classic "Bug 'Mech" group that is supposedly ubiquitous (the third being the Locust - I'm very happy that one made it in!). This is true even more so out in the periphery and yet more so considering they are built by virtually all adjacent powers.
- The Valkyrie is similarly ubiquitous in Federated Suns space (almost a signature 'Mech), and would have disseminated into the Aurigan Reach
- The Phoenix Hawk is an ubiquitous medium 'Mech available pretty much everywhere
- The Archer is an ubiquitous heavy 'Mech available pretty much everywhere
- Among the heavy 'Mechs, the Crusader and Warhammer are missing but neither design is particularly ubiquitous so this is an edge case (and the Marauder is even somewhat rare)
- The Longbow is an ubiquitous assault 'Mech available pretty much everywhere
I would generally defer to the fan-produced
Xotl RAT tables (these being unofficial, but created with vigorous input from the community's lore hounds; see
here for more info), and use the Davion and Liao tables as a base with the caveat that heavy 'Mechs would be rare, and assault 'Mechs almost nonexistant in the periphery (to say nothing of repair and maintenance status). Mind also the domestically produced 'Mechs of the Taurian Concordat and the Magistracy of Canopus, as listed above by
@Lord_Grayson.
At least according to early sources, assault 'Mechs made up only around 5% of all 'Mechs in the 3025 era timeframe.
House Liao had lost the capabilities to manufacture assault 'Mechs altogether, and could not build 'Mechs exceeding 70 (iirc) tons anymore either. Which is why they came up with the
Cataphract, essentially a FrankenMech-turned-line production model design that was a viable all-purpose combat unit built from proven, domestically produced parts only to shore up the Confederation's 'Mech roster.
