Breeding/sexing plecos

Depends on the species. With ancistrus, it's usually easy as the males get larger, knobbier bristles, while females either have none, or very small ones.

Check out Planet Catfish for species specific details. I know with some species it's very difficult to sex them, especially as juveniles.

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.