I have two, both females the same age (now almost 14). The first was planned and then six months after I got her, the second one was dumped in my neighborhood. I think two years would have been better for the spacing.

They used to play with each other a bit more when they were younger, but it was never an everyday thing. Even though they can be bratty to one another (okay, the border collie is a brat to the mutt), I do think they are bonded to one another. The mutt to the border collie more than the other way around, though.

I have heard that dogs of opposite sexes get along better. So the fact that you're considering a male is probably good. There was a period of time where I separated my two dogs when I left them alone because they were fighting; it lasted about a year or so when they were about 8-9.