North American 19th century cavalry officers?
I see the flaw in my logic
Well, the reasoning for excluding north american cavalry officers would be that their technology (industrial manufacturing of uniforms and weaponry) is too far beyond that of the medieval-centric tech of calradia/m&b. Classical Roman soldiers existed in a different time period, but fit technologically. Better tech than viking/barbarian/searaiders, but less tech than 16th century armies (basically). The same reasoning could be applied to ninjas, samurai's, mongols, ancient greeks, etc.
Generally, cultures that fought with pre-industrial technology will fit.
In theory, you could include cultures of any technology, and create some sort of system where higher technology comes at some cost to the player. For instance, unlocking tech with generations.