Honestly, if you're not looking to upgrade that tank within a few months (pbass grow uber quick) I'd leave it at the one fish. I'd still upgrade the tank, but wait until the pbass is out of there to add other fish. If you just have to have some more movement, try to grab some other Pbass, of the same variety. As I understand it, they will color up better with others of the same kind, and it might even encourage them to grow faster.
Without the Pbass, I'd probably go with some cool catfish, and maybe a Sanjica Cichlid.
Best of luck m8