These are beautiful fish. With an aquarium your size, I'd be willing you could keep two of almost all the dottyback species. You've got plenty of rock for hiding spots also.
You'll have to be careful not to get two males if you do get another. At two inches, it may still be female or sexually immature. I know most species of dottyback change sex from female to male (the opposite of clownfish). Some of the species are capable of reverting sex and going back to female though. It's pretty cool. This takes about twice as long as the change from female to male though.