I agree with Teal, that bully breeds and herding breeds don't mix well! My BC hates bully style playing and mannerisms. He'll just snark the living hell out of them.
I find that a lot of people have herding dogs and toy dogs and they do great together. Herding dogs lack true prey drive (or, the part of prey drive that leads to maiming and killing) so they tend to be very gentle with small dogs, and most herding breeds are sensitive and enjoy playing with smaller dogs more than big, physical dogs. Herding dogs usually get along with terriers too, as they'll back down to a tenacious terrier and aren't much for fighting, but they love the spunk of terriers.
Sporting breeds and hounds usually get along smashingly! Sporting breeds also do well with bully breeds, as they both love to play physically with body slams and rough wrestling.
Breeds that I've seen do well in pairs: Corgi/GSD, Corgi/Border Collie, JRT/Beagle, Husky/Border Collie, Collie/Greyhound, Bulldog/Chihuahua, Westie/Corgi, Bloodhound/Lab, Pit Bull/GSP, Pit Bull/Rottie.