I would add a school of something mid-tank. Perhaps Tetras, to go along with the South American theme you have.
How about something a little bit different like Black Phantoms? They are handsome fish.
They are alright but I would like to have some decent sized mid or top level swimmers which are about the size of Bolivian Rams. I was contemplating some sort of gourami but apparently they have terrible immune systems and are likely to get sick and die.