I think a Midas would be OK in there.... you got plenty of rock for him to hide in in case he feels threatened? I've got a 28 cube with an ocellaris & six-line and was looking at adding one to my system as well......
If you do still want to add one, wait a while on the royal...... let the system adjust to the extra bioload for a little while, and let the fish adjust to each other. Since I have a lot of coral, I think 3 fish are going to be my limit, just can't really nail down what I want..... too many cool choices.