Goldfish eat smaller fish as long as they fit in the goldfish's mouth. Goldfish do not have a picky diet. They eat a lot and go for whatever they can find that will fit in their mouth. This includes everything from fish flakes to other small fish.
Goldfish also enjoy eating pellets that are sold in pet stores, as well as boiled vegetables, peas without the shell, brine shrimp and bloodworms. All pieces must be small enough to fit in the goldfish's mouth.
To keep other fish in the same tank as a goldfish, the other fish need to be fast swimmers and larger than the goldfish. It is also important to remember other rules for having multiple fish, including choosing fish that like the same temperatures and that do not eat goldfish.