I agree that a smaller juvenile perc should be added. Adding a new clown that is bigger might cause some issues because your existing clown might of gone thru the sex change and became a female. It only takes about a month for a sexually mature clown to turn into a female with no other clowns in the tank. So choose a juvenile clown they are still is sexually immature.
As for firefish, IMO
they are boring and seem to have almost no personality when compared to a perc or an ocellaris. I almost regret getting my firefish but it does add some diversity to the tank