Hi Jay,
If your thinking about adding a Parrot to you 55g then I would say your probably best of leaving it..
If you really want to get another few fish I would advise you to build on you Tetra's etc.. as you only have a few of each or maybe another platy to go with you other one.
Just a note: In your sisters tank, it say's she has a Pangasius Shark.. Does she realise how big they can get?
Whilst this catfish is fairly peaceful, they are best kept away from small fish as they will eventually appear on the menu.
Best Regards
Lawrence