The Pyramid Butterfly fish (Hemitaurichthys polylepis) is one of my favorites. If you are able to land a healthy trio for your reef they make an awesome display fish. They are about the safest you get with butterflies in a reef. They are hardy once established.
They will readily eat prepared foods including frozen mysis, cyclopeeze, fresh shrimp, pellets, flakes, etc. With any fish, a varied diet is best. They usually max out at about 7 inches.