I think you should be cool gettin a couple of clowns They grow 2 6" in length.
They appreciate a nice current in the tank as they originate from Indonesian streams and like to have a place to hide and sleep in.
If your using the old "1inch of fish per gallon of tank rule" you can predict that you could house 6 fully grown clowns in a 38 gal tank. This gives you space for another 2inch fish!