Great to see another Canadian frogloverwelcome to the Forum.
I don't suggest inter-mixing frogs, but you can if you want.
That's a tough one. Northern Leopard frogs might be an option, though it might be a bit small... some kind of Ranid might be good. NOT an American bullfrogs. The froglets/juveniles of that species require a 20G+, let alone the adults. Possibly a wood frog or smaller kind of Ranid?
Treefrogs are good, but if you want a good swimmer, a Ranid would do. You could try a Northern Green frog, Also known as Rana Clamitans Melanota. I'm not sure where you'd get one, though. Give me some time to think about this... maybe some other, more experienced frog owners will comment and give you their suggestions.