I just spent the last month researching tankmates for my ryukin, and settled on a small school of pearl danios. They're still in quarantine, but I'll be happy to let you know how it goes in a couple of weeks! I know a lot of people do the white clouds, but they start out so small, I'm scared they'd get eaten. I'm hoping the danios are big and fast enough to work out (plus there's a ton of hiding places in the tank to help).
Speaking of eating tankmates, originally I thought for sure peppered cory cats would be perfect, but later learned there is a pretty large risk of the goldfish trying to swallow the cats (either trying to eat them as is, or cleaning up any deceased cats) and having them get stuck, possibly killing the goldfish. With a larger tank, weather/dojo loaches sound like a pretty good bottom feeder to me (I particularly liked the look of the golden ones).
