Mine is not aggressive at all. He was about the size of a quarter when I got him. Also, very tiny. (Not as tiny as a fingernail, I would have loved to see that!) He is actually very shy and has a specific hiding place that he goes to. I really like him.
I feed him seaweed strips, mysis, and the occasional pellet food soaked in vitachem.
My advice is to make sure you QT a hippo tang. I know you should QT all fish, but this fish in particular is EXTREMELY prone to ich. Check out the disease section of the board, there is always a thread about hippos and ich.