Short of physically examining the fish for sex organs, there are not any external differences between a male and female pleco. They won't breed until they're fairly large--more than 6 inches, I think. Commercially, they are bred in large ponds.
I wouldn't bother--I think the commons would end up being a pain, just due to their size and the low demand. If you're interested in getting cats that will breed in the aquarium, look into bristlenose. Easy to breed, and there is some demand for these smaller, more useful fish.