Discus need to be in groups of at least 5-6 or a breeding pair. I currently have 2 juvies in a 55 g planted tank but there was no other option for these guys and they are hiding a lot unfortunately I can't get hold of any more the same size and adding ones much bigger would not help them.
As to non-discus tank mates cory cats, cardinal tetra (they tolorate the high temp better than neons), bristle nose cats etc I wouldn't get black tetra they can be fin nippers and whatever you do don't get tiger barbs.