I've always been partial to same general species in a tank. For example, I have a Carpintis and a Gold Saum in my 55g. Not the same exact species, but are very similar in area, temprament, and feeding habits. But if I were you, I'd just go with a single red or jag. They're both beautiful, very aggressive, and are great as a single species. If you want to go down the road of pairing, I'd say get a pair of JDs, particularly EBJDs, although the price tag will be a bit high.