Utterly Mad

I would've assumed Zidians kept mechs to themselves, to give them a combat advantage and only rarely allowed other races to use less advanced mechs than their own. For example the two that they allowed SiegeWorks to produce (I'll get to work on them soon)

Maybe a bit more in-depth Description and History, feels a little too short, give them some character, set an idea for what they've been through and how it's shaped their business practices. Work some stuff about other companies into their lore, how they've interacted with each other in the past. For examples of things that might be interesting, SiegeWorks has two mechs that they're building that they were licensed by the Zidian Military to mass produce, perhaps Blackpolis was competing with SiegeWorks for the military contract and simply lost due to production scale? Maybe something about how another company did some shady stuff trying to steal from Blackpolis (probably SiegeWorks, as usual) and another company actually stopped them in return for a trade agreement (probably Lork-Tech)

A Mech specialised company is a great idea though. In regards for the weapons built by other companies, it may be best to see if they're producing any mech specific weapons first and then add them in later. The SiegeWorks Light Lasers will be produced, probably from stolen plans for the Lork-Tech Heavy Laser if Lork-Tech ends up producing one.
The idea of using other companies tech is a great one though, allows the companies to work together in some ways to produce weapons for Blackpolis instead of just competing with each other all the time.