The goldfish and koi make it kind of tough because they will eat most smaller fish especially juveniles.
If you have a way to section off the tank you could breed some killi's or the long-finned or yellow WCMM. You could also try to breed high value koi or goldfish if you have time and the money to buy the initial stock. But again you will need a way to isolate out the fry from the parents and your selected parents from the rest of the tank.