What would be a good "Empire" kind of race?
Cathar are force users so make Jedi Knight and Sage and militant so Trooper.
They can be slaves (though apparently Males too difficult) so can be Sith Inquisitor and that is about all I see fitting, before legacy unlocks.
What would be a good Empire counterpart?
Devaronian. Or, like Veirata said, Falleen would be an excellent choice.