First, it's not a beat 'em up. It's a fighting game. Gawd, will people ever get the genres right?
Second, Carmilla was never really given any special characterization in the series, so it's not like they had anything to go by. Besides, what would you do with a vicious lesbian vampire? Turn her into a lady? I think not.
Third, about the variety...I'm pretty sure if you, or someone like you, was in charge of the characters there would be very little variety, what with wanting Richter and Soma in the game. Like I said before, they managed to keep the cast at a minimum while also giving it a lot of fanservice and variety. Think of it for yourself, will you? They had a limited time to create this (they always do, Konami pressures KCET, the team that makes Cv, a lot) and, for what it's worth, they did an excellent job with the cast of characters, and the game.
End of argument.