well 20 is a pretty small tank but any smaller fish would be good firefish clowns ect ect, tangs are pretty much out of the question even tho ive had a few in a 30 dont tell anyone people in this forum will freak out if they knew.
Firefish are a good choice but there are alot of other gobies you can get, clowns are just so popular its almost weird to see a tank without one. you could also look into damsels or maybe chromis just be careful some are agressive.
yea there's alot of gobies/blennies that stay small like watchman,scooter etc. but my advice to you would be to not even start such a small tank just get a good size one to start because it will end up being much easier in the long run. You really have to watch your water quality and test often on a smaller system because you can have a very bad situation very quickly. Best bet is start a larger tank w/ a sump so you can put macro algae like chaeto culerpa etc. they help control phosphates and will benefit your system greatly.