All of the above, ESPECIALLY if you rescue him/her from a tank that was never fed for about six months.
They get starving kitten syndrome...we have one in a ten gallon that is VERY well fed by us, but he STILL goes after the one poor chromis we have with him, snails, occasionally a small group of button polyps, etc.
We set the tank up from stuff we just had laying around especially for him/her, so it's not that big of a deal. But I would think twice before sticking one in a well thought-out reef tank.