Generally goldfish are social creatures, so I wouldn't keep one on it's own out of choice.
Fancy goldfish all grow to roughly the same large size which is around 8 - 12 inches, depending on breed.
Single tails I would not suggest for a tank, they fare much better in ponds, as Vic says she had to re-home hers within 6 months.
If you really, really want to keep goldfish, perhaps you could save up to get a bigger tank
You could always sell or trade your 30 gallons with someone, for something larger, second hand websites or other fish forums may be useful there.
As for other fish with 30 gallons, I would suggest checking out this website; (Link is not visible to guests. Please register to view.
If you in put your tank and filtration details then click 'Display only suitable species for your tank' you will be surprised at the possibilities. A tropical tank is always a possibility, you'd just need to add a heater.
Vic and Scrivens and a few other members keep other fish so could advise you best on the smaller species of fish
Vic has platys like a few other members here and I know Scrivens has a tropical set up but I forget what fish he has