Hermit,
If your Percula is a juvenile, another juvenile Perc should be fine. Eventually, the dominant one usually becomes a female and you would have a pair. If the Perc is larger, get a smaller one to avoid the same territoriality you are having now.
I would recommend getting a copy of
Joyce Wilkerson's book, Clownfishes. It tells you just about everything you need to know about clowns.
My
Ocellaris pair have used about everything in the tank for shelter, including mushrooms and my Frogspawn. Mushrooms would be a safe bet, especially if your tank is young.
HTH
Dick
