I've also heard that dogs get along better if they are the opposite sex.. But I have had two females in the past and never experienced any problems.

I have also taken care of a male Cavalier King Charles Spaniel once when I only had my female Jack Russell and they didn't get on at all. He wasn't much of a gentleman I'm afraid.

At the moment I have my JRT and a male Border Collie mix and they get along superbly. They play together and my JRT has gotten a lot better with her anxiety of being in the car since he got here.

So yeah. I guess what I am trying to say is that dogs don't automatically get on well together just because they are the opposite sex but it probably has more to do with their individual personality.