Size certainly does not come into it - the smallest of dogs can decide it is going to be top dog, and then you would have problems. I would get a male and have it castrated if the bitch is not spayed, although I would spay the bitch to prevent unwanted attention from neighbouring males.
Dogs get on better than bitches, if two bitches decide they don't like each other they don't forgive and forget, they just wait the next opportunity to get at each other.
Two 'bull' breeds could be a bit of a problem, as they are both rather headstrong although the Frenchie would probably be a bit more amenable.