I would stick with another clown, a firefish, or a royal gramma.
I plan on getting 2 percula clowns, 1 firefish, and 1 royal gramma for my 38 gallon. All three are very hardy, easy to keep, peaceful and they all look great. And I would think 3 out of the 4 could be housed in a 20 gallon long as the cycle was done correctly.
38 gal SW
, 40lbs base rock, 5lbs live, emporer 280, bio-wheel 330,tronic heater, 2 O. clowns, 1 firefish.
29 gal FW
, boring...soon to be salt I think.
10 gal SW
, 20lbs LS
, 20lbs base rock, 1 unhappy domino damsel (he was in the 38...switched the percs and the damesel recently).