Please answer the Tank size question. Most everyone here would hate for you to doom any fish you decide to get by throwing them in a tank not suitable for two fish. You realize that, in a small tank, your established RD will not allow any fish to recover from the initial stress, right? so you can get a Dovii and it will get killed. Also, two aggressive fish in a tank by themselves are very difficult to manage because the aggression will not be spread out. One will eventually kill the other.
If you do have a large enough tank, many of the fish mentioned on this thread would be well suited for your RD.