Mainly just goldfish. Specifically goldfish of the same body type, i.e. fancy goldfish with other fancies, goldfish with impaired vision with others who also have trouble seeing, comets with other slI'm bodied goldfish.
As far as other fish go, FANCY goldfish can be kept with white cloud minnows, dojo loaches, and sometimes guppies, though I've never personally tried any of those myself. I wouldn't keep those with any slI'm bodied goldfish on account of the fact that they would eat them.
A. BorelliI can sometimes work too
Anything peaceful and not small enough to eat. Naturally water parameters need to be same.