5 gallon tanks are pretty good for quarantine tanks. They can be used for bettas although they do better with more room to swim around.
It's usually recommended that the first goldfish has 20 gallons of water and each that is added have an additional 10 gallons of room. The reasoning for this is because goldfish are such heavy waste producers.
Actually I was looking for advise on fish other than goldfish or betta for a 5 g tank ;D. I've been having too much fun with my tank so far with 4 neons and 2 ghost shrimp I'm already thinking about upgrading and it's only been 2 weeks! I really am fasinated with the cichlids but I'm not ready for a 55 g tank just yet...but I'm getting there! Right now I'm probably full although the water looks great and the fish look good except one bully Tetra. I thought about putting 1 albino corydora in there but might be pushing it (besides I guess they don't do well alone)...what do you think?