It will take new fish a few days to acclimate, RD's are great fish as you can see from my profile pic I keep one as well. They are highly enteractive as well. They are also Highly highly aggressive. Chances are if you don't have a male/female one will be killed off by the other.
As far as tankmates. Depends on the size of the tank as its is said the minimum for a single fish is 55g. These fish can reach up and over 12 inches.
Unless you have a very large tank 125g+ I doubt tankmates are gonna be possible.
However... There are exceptions to every rule. You may end up with docile ones who don't mind other tankmates. However that would be more rare as they are named "devil" for a reason and generally won't allow tankmates.
Good luck, don't let the aggression scare ya they are great fish!