The Kenyi are going to present a problem. Adding more is important to distibute their high aggression and eventually if you just keep two you can be sure you'll end up with just one. Your best bet is to get rid of them altogether and go for milder fish to avoid inevitable future fatalities, but that's your choice.
Either way, you do have room to add several more fish. Except for adding more Kenyi (or getting rid of them) you have a lot of choice in what you can add, and it largly comes down to personal taste and availability. Just don't get anything that looks like something you already have, and aim for groups, not pairs.