Hi there! Can I ask what size tank is the RS in?
I have a Rainbow Shark so can give you bits of advice based on my experience over the last year with him. I've had to move him around several times due to problems he caused!
1) They are territorial and do like to have their own space in a tank - by space I don't just mean a few inches around them, they require quite a large span of personal territory and will definitley chase any fish that enters his "zone".
2) If they are in a peaceful community tank, they will tend to be "The Boss" and rule the roost. They will chase the life out of some fish and at feeding times will try to prevent other fish from getting the food by bullying them away. This can lead to other fish become stressed and sick.
3) If they are in a tank with other semi-aggressive fish of a higher ranking level (e.g. convicts), you may see the roles reversed with the RS being the one that is bullied and it will usually retreat into a corner or hidey hole and keep himself to himself.
4) When introducing new fish to your aquarium it might be best to remove the RS to a spare tank or hospital tank whilst you rearrange a few things in the main tank. Add your new fish and let them settle, then add the RS back into the tank. The RS will then have to set new territory based on the fish already there and as he will feel like the "new kid on the block" will be less likely to target the other new fish in the tank - well, at least not until he's sussed out whether they will fight back or not.
To be honest, they are not the easiest of fish to keep, purely because of their temperamental nature and incessant bullying (they are not so bad when they are juvies, but as they reach full size that is when most problems occur).
I wish I had known all of this before I went ahead and purchased mine because he caused havoc in two tanks - and now that he's in a larger tank with just two small cichlids, he is now the one in submission (and I feel he's not entirely happy about that!) and I know I'm going to have to rehome him soon, which is a real pain.
Regards - Athena