These guys live in my "backyard" which is a COLDWATER ocean, so unless you are keeping them in coldwater I would not recommend you owning own.
But say you do have a coldwater setup you would want a 150 gallon tank since they get 15 inches in length. Juvenile Garibaldi are not as bright in color, but have blue spots which they lose as they become adult. like all male damsel fish, the male Garibaldi will boldly attack much larger swimming creatures so you are probably safe since In a 150 gallon tank you won't have larger fish.
But I have heard of people having them in tropical setups ( not recommended )
p.s Butterfly's would not work unless you have a massive tank so the garibaldi's aggression will be "masked", and butterflys are not coldwater.
Hope this helped