Unlike many others, I've had better success with fish that are smaller, not the same size. It is easier to create hiding spaces if the aggressor is much larger than the submissive fish, and the aggression tends to be less IME as the big fish doesn't feel threatened by the smaller one.
I've found that convicts are a great choice for larger aggressive fish. They are smart, fast, and tough. Give them a bunch of caves and they'll fit in just fine (assuming they survive the first few minutes!). Introduce them carefully. The best thing would be to take the RD out until the cons get established -- 20 minutes or so.