You would probably be ok with keeping 2 pearl-scales in a 20 long. they tend to stay a lot smaller than other goldfish. in this case, however, bigger is always better!
Anyone else feel the Yahoo Answers Deja vu feeling.. lol..
I myself had one of those common fantail feeder fish in a 20 gallon tank all by himself and lived for about 10 years. But this was before I went hardcore and actually was learning how to take care of fish properly. I would say at least a 29+ gallon for a goldfish along with dual filters.