Some people say that two girls will fight. I have four girls (no boys) and we've never had any issues.
It will take a little while for them to get used to each other. They will eventually come to love each other even if they don't hit it off right away. I think your best bet is to choose a dog that has a similar profile to your current dog. IE, if Molly is rather laid back, don't go get a jack russel
