If you're going to use the 20gal here's what i would recommend:
2x firefish (purple or regular)
a pair of occelaris clowns. DO NOT buy the bonded pair bullcrap. TOTAL WASTE OF MONEY, if you buy 2 occelaris clowns they will eventually become a pair as they are hermaphrodites.
1x Hi fin banded goby.
that stocking would give you some nice colour and a low biological stress on your filtration.
But as previously stated "in saltwater the bigger the better" If you are interested in getting a bigger tank, here is a few stocking lists for some regular choices of tank sizes.
2x maroon clowns
1x dwarf lion
1x flame angel or any other dwarf angel to your liking
or a more peaceful
5x reef chromis
3x green clown gobies
1x snowflake eel
1x pinkface wrasse
2x dwarf lions or 1x radiata or russels lion
1x picasso trigger (PLEASE introduce last as triggers are agressive to newcomers on some occasions, or on others can be happy additions to a tank. If I were you I wouldn't take any chances)
or another peaceful
1x kole eye yellow tang
1x fairy or flasher wrasse
4x reef chromis
1x dwarf angel
a pair of occelaris or clarkii clowns
hope I helped and gave you some ideas, let me know if you decide to up your tank size or keep it the same. Also be sure to keep us postewd on future progress