*nods and agrees with Barney and d9hp*
Water changes are critical. I've a 29 gallon with a pleco, 3 angels and 2 loaches. I do 10% water changes/gravel vacuuming 3 times a week! When you deal with messy fish (like my plec...the poo machine LOL or goldfish) you HAVE to get rid of the waste. Keep in mind, anything that goes INTO a tank does NOT leave unless YOU take it out.
Keeping them in a small tank WILL stunt them. Prob is, as was mentioned, you will deform them. I guess what happens is the body size doesn't change as fast as the organs. So the fish die younger and don't live out there expected life span (which on gold fish can be 20+ years if treated properly! Check this out:
http://www.goldfishinfo.com/goldlife.htm . You can see how big they get and how long they can live. If you want to read more about goldfish, click the link at the bottom of that page that will return you to the main page of the website).
To be honest, I would prob put 2 of the goldfish in the big tank, and keep one in the 10. I'd also suggest looking into some small bottom feeders/algae eaters as well, BUT, they would have to handle low temps, as goldfish are NOT tropical fish and don't do as well at tropical fish temps (like I wouldn't recommend a pleco cause they come from rivers in South America and live in warmer temps).
Of course, the final decision on what to do is all yours, but it sounds to me like you want to do right by your fish
Thumbs up to ya