some people may argue this, but................you could try a jewel cichlid or two. there is a guy on here that bred them in a 10, so they should be fine.
I know exactly the answer: The Amazon Leaf fish. You could probably keep a couple in a 10g, and it must be very well planted. They can easily swallow guppies.
I feed all of my livebearer culls to my convicts. It's only fair since ALL of my convict fry are eaten by my other fish . The cons arent that big themselves but they are easily able to snatch up even fry that are weeks old.