i started with a 20 long, an i had 2 occelaris clowns, 1 yellow tailed damsel, and a coral banded shrimp. however, knowing what i know now, i would recommend:
1 true perc (since you want one)
1 sixline wrasse
1 lawnmower blenny (to help out with the inevitable hair algae outbreak)
and 1 peppermint shrimp, just because i think theyre a good thing to have incase a rock happens to bring in aip,
or a cleaner shrimp, whichever you prefer.
in a 20 long you could probably add another small swimmy fish, like if you wanted a pair of percs instead of just the one. (theyre so cute!) of course, you should be running a skimmer and check your levels often to be sure your not overstocking your tank.