I use Rescue Remedy all the time. Pete and Onyx pick on Ivy something terrible. At my vet's suggestion, I add rescue remedy to their water every day. It has greatly reduced the picking on Ivy. I use it alot with the dogs too--Pingo gets it if I know there is a storm coming, Kayleigh gets it before she goes to the vet.

Have you tried Feliway? I have the diffusers throughout the house. It does seem to help both with inappropriate peeing and aggression towards each other (well, mainly aggression towards Ivy. The Princess is a target for the other cats!).

The bonus of Rescue Remedy is it works for humans too. I've been known to dose myself up before taking Kayleigh to the vet!