We got our fist kittens last year. I asked the breeder if it mattered - 2 boys, 2 girls or 1 of each - and she said it really didn't matter, they would either get along or not. So I picked 2 I really liked, a boy and a girl.
They get along really well, although as they get older they seem to be ignoring each other more!
Took them to the vet to get them done, turns out little Echo is actually a girl
so I can happily say that 2 girls get along fine!!