I would recommend going to the fish store, having a look at what there is and what takes your fancy. Its difficult for people to make recommendations, because what you get is what appeals to them, not what appeals to you. Whether fish x goes with fish y is another matter, that's where experience comes in useful.
A school of tetras generally will go with your other stock plans, but there are plenty of options. Are you looking for something colourful, or active, or unusual, or with interersting behaviour?