Part of Traviss' issue was that she made some hilariously inaccurate estimations of troop strength for both the GAR and the Separatists
Was this before or after that troop strength quote from Ep 2, which only makes sense if "unit" means some group of dozens of troops, but is never concretely defined (but is absurd if it's meant to represent a single trooper)?
In any case re: Mandos. I think the fact that to many fans they are a pan-Species organization means that having a Mando "species" isn't a great idea. If anything, Mando should be a template (in the same vein as D&D's Dark or Phrenic or Half-Minotaur), but that's a mechanic this game doesn't really have either. So with the mechanical options there removed, I say go for fluff, take an Oath to the Resol'nare, and just act/dress/arm yourself like a Mando would.