Looks good, but...
I've never kept a cardinalfish at home!
My favorite saltwater fish family! Give them a try some time
Anyway, a few specific notes...
Blue eye Cardinal, Apogon leptacanthus (2.5")
I've seen this fish kept in groups in large tanks. Not sure how well it would do in a smaller tank.
Just about the perfect fish. Also called the glass or threadfin cardinal. A true schooling fish, it will school in even small tanks. They are shy but gradually gain a bit of boldness if nothing in the tank will bother them. Hearty eaters, hardy fish, breeds easily in captivity. A group of 3-4 would definitely be okay in a 20 gallon.
Other small cardinals suitable for a nano are the Pyjama cardinal, orangelined cardinal (another schooling fish), any of the luminous/arrow/mini cardinals (small schooling cardinals under 1" TL, such as apogon neotes or rhabdamia gracilis) if you're lucky enough to find a source of them. The conch fish (a cardinal that lives inside of conch shells, even commensually with live conch) would also be suitable, just a bit cryptic.