i think getting a male is a good suggestion, depending on how dominant your collie is. My two females got along very nicely for a couple years, but when my rott-mix female matured, they had some very nasty fights. Of course, other times they were best of friends. It was very sad that she got hit by a car when she was only 3, but then we got a male because two breeders told me females can fight really nastily and often.
Everythings been quiet ever since!
I really think it just depends on their personalities. It's wonderful that you're getting a second dog!