Convicts definitely have potential. You could also breed paradise fish (Macropodus opercularis). They spawn with extreme regularity and they have large hatches virtually every time. Also, being a labyrinth fish, they will tolerate some degree of overcrowding much better than convicts.
They are however a smaller species than convicts. Just some food for thought.
definitely convicts since minnows and goldfish have thiaminase and generally speaking predatory fish such as convicts have more protein and are healthier