I actually feel the same way as duanes, in that biotope tanks are best, and I have in the past been in a position to get everything right for a biotope (correct fish, plants water etc.), while I was working in Belize, Botswana and Brazil. But now I live in the Andes and I am largely dependent on what the fish stores sell so its not very easy. However I do stick to fish from the same continent and with similar habitat requirements, even if they would not necessary occur together naturally. In your situation I would actually choose some larger South American tetras in place of the Congo tetras. But Congo tetras should be fine if you like those best.