I am wondering would there be any problems in adding a yellow tail to the tank with a pair of established perculas? If there is I'd like to avoid it than.
Unfortunately, "Yellowtail" the common name represents a few fishes. If you mean the Yellowtail Damselfish (Chrysiptera parasema) then it probably won't be a particularly good idea.
Anemonefish are a type of Damselfish. The established Anemonefish will probably not accept the introduction of another Damselfish.
If this isn't the fish you had in mind, you'll need to provide me with the scientific name.