If you have a single percula, depending on the age of the fish, if it has reached sexual maturity yet or not.....(have you had him/her more or less than a month?)
Keeping a single specimen will in all likelyhood end with you having a female percula, or a "nemo-ette". This being the most likely case, you'll want to find a juvi percula to add, one definatly smaller than current occupant. This will increase the odds that you will end with a pair of clowns as the perculas (like so many others) are hermaphroditic. You'll have 1 dominate female in a group of clowns, 1 dominate male, and then a bunch of subordinate sexually unmature males. If somethign happened to the female, the dominate male would "convert" to female, and pick the next dominate male. Once they change to female they cannot change back to male though.
The only reason I'd see them fighting over the anemone is if you ended up with two females or two different species.