Normally they can live in that condition fine for a while, given that you make sure you do regular water changes, siphoning, added bubblers, etc.
The stunted growth thing only happens when the fish has to suffer in a high ammonia and nitrate environment. They've done studies of using 3 inch diameter tubing where they would have (bass?) grow from babies in there. They would continuously pump in clean water in, dirty out, and those poor fish grew to the size of those tubes. So it's not necessarily a "Size" issue, so much as that goldfish are really dirty and need the gallon amount to make it so they don't dirty it up in less than a week (hours for those who put them in bowls). That, and they do enjoy the swimming space.
I think a 15 gallon would be good for now while he's small, but don't add any more in there to be safe and you'll be fine.
At least you're not asking us to put 4 goldfish in a 10 gallon, like one of my customers today
Poor things.