Well most any dog book you read will tell you that opposite sex pairs are more likely to get along than same sex.
I also read in a lot of books, and on a lot of websites (with people's personal accounts) that female/female fights are usually much worse than male/male. Of course there are always exceptions.
I haven't had any problem with my two female/male, but I wouldn't expect any problems with either of them if I brought another dog into the house, no matter what sex.
What matters most is the temperment of the dogs in question.