don't worry, you'll have no problems with introducing a new clown, just make sure it's much smaller than the one you have.
Again not really the case. I lost a clown due to it just not taking food. The pair (both true percs) I had were both around half an inch long. I then added another Trure perc much larger (around 4" long as it was the only size I could get) with the half inch fish and they paired up straight away, even while the large one was acclimatising the tiny clown was up and swimming by the bag! I've had no aggression issues, the little one follows the big one around the tank and the big on has no problems with this.