Well, for the most part, most mass production units are fine. There are certain units that problems have been expressed with (Astrays, Strike Daggers, the Zeta Plus A1s), but they're largely exceptions. Things like GMs (And their many variants), Zakus (And their many variants), Goufs, Flags, Leoses, Jegans, et cetera are all pretty much allowed, barring certain extremely customized variants.

The first few of those actually have bits of backstory for their role in this universe prepared, but little is set in stone.