I manage a LFS, and talk to wholesalers all the time. When we first started bringing in these tiny hippo tangs, we were losing them at an alarming rate. Then we figured out that they're like new born humans. They need to feed almost constantly. Hikari makes some tiny seaweed pellets that work great for this purpose. I just keep a few of these on the bottom of their cubical at all times. We basically don't lose them any more.
I wouldn't worry to much about aggression early on. The hippo will hide, or hang out right outside it's hiding spot most of the time. When other fish come by, it will dart back to its hiding spot. Typically, other tangs aren't very aggressive towards hippos. Hippos are in a genus all to themselves with no close relatives. Usually, other tangs don't seem to care about hippos at all and seem to just ignore them. With that said, you do have a couple of notoriously aggressive tangs that have been in the same "smallish" tank for a long time. Adding any fish can be risky.
HTH
Peace
EC