What fish can I add to this aquarium (if any)?

Get more kuhli loaches and get rid of the angelfish when it gets big. Don't get the pleco, it will overstock your tank. Sparkling gouramis are the more peaceful gouramis but the dwarf gourami may become territorial. If you had less livebearers (maybe when they die, or you move them) you could have enough space for a school of tetras. You could get a 2 schools even, and your tank would be fully stocked then. Good luck!