This is not in the general sense a community fish, they have a reputation for not playing well with others. Out of all the Midas complex species Ive kept and I have not kept them all these are the most aggressive in my experience. Some fish grow into their aggression RDs have it from the start. I was growing a small Labiatus out in with my GT for a month while moving some fish around and this little guy all of 2.5 would flair and pump and threaten my 7 GT, it had no fear. VRWC is correct, without a very large tank and very robust tank mates the likelihood of successfully housing an RD with other fish is very slim at best. This fact combined with the longer this fish is housed alone will make its acceptance of anything added to the tank highly unlikely.
You can try Silver dollars go to Petsmart they have them right now for a buck, get 7 and see what happens. Also try putting some type of cover in a corner so it can retreat behind it when it feels necessary. I have a big fake rock ornament in a corner with a fake vine suction cupped to the side of the tank above it for additional cover, my RD likes to go behind it once in a while, gives them a sense of security and calms them down a little when they gat too rambunctious. Every once in a while they go thru what I call a Rage where they kite and flair and attack things in the tank for what would appear to be no reason to us, I suspect some movement in the room was not to their liking, when they get into this state and other fish are around bad things happen. So there it is youre probably stuck with a solo RD or start over I guess.