THe heniochus will for sure eat a bunch of things you don't want it to in a reef type aquarium. THe long nose and copperband aren't as bad about eating desirable inhabitants but often eat small inverts (polyps, feather dusters, etc.)Many people include Long Nose & coppeeband butterflys in thier reef tank for removal of aptasia anemone's.
Non of these fish belong in a 30 gallon......
As per "what is considered a reef tank", I believe this will very from one person to the next. By and large, I think most people would agree that it is a system that includes live rock & live corals but much past that, the defination of a "reef tank" may have many opinions.
My defination of a reef style aquarium is a system that mostly relies on live sand, live rock and other naturally occouring organisims that mostly perform the water processing function. I other words, I don't rely on man made filtration systems to maintain my reef tanks other than a protien skimmer. THis is a very large subject that would be very lenghthy to cover in this forum. If your truly interested in doing a microcausim get a good book on the subject. There are many available, I'm partial to Dr. Walter Addey's "dynamic aquaria" but several other are good reading too.
Joel