I'm not super experienced with butterflies, since I just got a bunch this year and had one last year, but before adding them I want to make sure my coral head is so big and growing so fast, that a butterfly picking at it won't damage it. My experience with fish is that if given a choice, they will eat the yummy food you throw in as opposed to the tiny bits of coral flesh they can pull out of their skeletons. You need to train it to frozen clams, shrimp or whatever before introducing into the main tank.
This is a famous post by lemonlemon I've linked here, which showed me what "reef caution" really means. After studying this, I went ahead and started looking at butterflies seriously. http://www.reefcentral.com/forums/showthread.php?t=1983397