I don't think that re-arranging the decor will "reset" who the boss of the tank is in this situation. From what I've heard, and what others on the thread are saying, RTS are relatively aggressive and very territorial, and the kissers are normally only aggressive with their species and pretty calm with other breeds of fish. I had a kisser for about a month before I realized she would get a foot long, and at that point all I had was a 10 gal, so I returned her to my
LFS. While I had her, though, she was great and I didn't see any aggression issues from her.
You may see an issue with the barbs nipping at the fins of your other fish though, so keep an eye out for that as well...I know that wasn't your question, but you said they were a gift so I wasn't sure if you had done much research on them. Also, I'm not sure once your kisser reaches the foot long mark, if it will try to munch on your platies. You may want to do some research on that before it gets too much larger.