How many koi do you have? actually, as the fish gradually grows, you can then upgrade to a tank (if you have the room and budget) to suit the fish, but you don't need a huge tank if the koi are still small, you can do the whole grow out cycle, people do that with all different types of fish, so don't get discouraged.
I own 4 koi and they get along great with Orandas, they're cute too and come in all sorts of colors. Then there are other types of fanned split tailed goldfish that they get along with. Larger comets are good, fish like that. Lionhead goldfish are good tank mates as well. Any sort of fish in the pond/goldfish family get along with koi, considering koi are on the more docile side.