goldfish should only be kept with other goldfish of the same type or else they will pick on eachother. They can be housed in the same aquarium with other slow moving coldwater species of fish. White clouds can be kept safely in the same tank with other goldfish and are a good match.
Goldfish must never be kept with tropical fishes. As it is from the Carp family, some fish from the same family will go well with a goldfish. Koi is from the same family and can be a very nice match. Other fishes that go well with goldfish are tench, orfe and rudd.
Weatber Loaches, chinese algea eaters, other goldfish, and white cloud mountain minnows. When goldfish grow large they will eat the white clouds. But all of these are cold water fish.
Any kind of cold, freshwater, peaceful to semi-aggresive fish can live together. There are numerous varieties of guppies, minnows, and other types of goldfish that can survive together.
Keeping a variety of goldfish types is preferable to trying to mix goldfish with tropicals. The water is harder to maintain a chemical balance needed for both tropicals and goldfish. There are many different types of goldfish that make an interesting tank display. If you would like another type of fish for variety, you might try a plecostomus.