If you want a surefire breeder go with Convicts....they are a no brainer.
In two years experience, orange marking mean males, no orange means females. Get one of each (makes no difference on their relative size) and put a large rock or clay pot in the tank. In my experience it takes about a week to get eggs from them. DO NOT put any other fish in the tank with them. if they don't kill them, the parents will at the least raise mortal hell in the tank.
Although not as uncommon, Convicts are by far and away the easiest fish to breed.