I have a 20g and it has: a striped goby about an inch and a half long, he's the largest fish I have btw, a two spot blenny, a barnacle blenny and a green clown goby and a yellow clown goby. Most of the fish are perchers or they hang out in the barnacles I have in the tank. The striped blenny is the only fish that actually swims LOL.
You can get gobies, firefish, cardinal fish, blennies, a royal gramma, chromis, clowns theres just so many hehe.
If you get a goby you can get one that can pair up with a pistol shrimp.