I don't think the sex of the dog matters. I agree with what someone else said about picking an older dog to match their personalities. They don't have to be the same age, but if you have a 9 month old pup, they might get on the other dogs nerves. Also, I agree with meeting on neutral grounds to introduce them before deciding to adopt. If you do adopt, I suggest keeping the new dog on a leash for a couple weeks even when supervised and remove all toys. When I introduced Kodiak, I had to do that as it is easier to break up fights. We had quite a period of adjustment.