Those are good choices; I've also found zoanthids to be pretty fool-proof ... and I've made some pretty foolish mistakes on my reef!
For fish, you might look at some of the goby/shrimp combinations that are so often fascinating - I've got blackray and yellow-watchman shrimpgobies in my tank, and quite enjoy them. Many of those guys stay quite small, so they fit well in a 20. Another thought would be the cleaner gobies of the genus Elecatina - neon, sharknose, yellowline are a few. Only one, unless you can find a bonded pair, but kind of charming as they bop about on the rock.
Tailspot blennies stay pretty small as well, and are _loaded_ with personality!
Firefish, which will hover above the sand & rocks, and possum wrasse, which will prowl between the stones, are also possibilities to consider.
~Bruce