If you have a good amount of room left in the tank (sounds like you do), I think your best option would be rainbowfish. They get to be a fairly good size, have beautiful colours and will likely school if you get a decent number.
Dwarf neons stay pretty small. Ornates, Australian, Bosemani, Blue, and New Guinea Rainbows get bigger, like 3 to 4".
Dwarf neons stay pretty small. Ornates, Australian, Bosemani, Blue, and New Guinea Rainbows get bigger, like 3 to 4".