i have three, soon to be four, not much aggression. as long as the yellow tang hasn't been in the tank too long n(more than a month) there wont be too much aggression, at first they might tussle a lil, but they will sort out who's the boss pretty quickly. anther way to minimize the fighting is making sure that the new tang is larger then the established one or significantly smaller so its not seen as a threat.
but my tank is pretty large so im sure that plays a large part in my success.
it can be done, just add them in at the same time. i have a sailfin, yellow belly hippo, blue hippo, purple, and yellow, tang in the same tank, they all get along, you just have to add them at the same time is all.
It all about body shape with them... I think youll be fine... I have 2 soon to be 3 almost like shape bodies and they seem to be doing fine.. Waiting for my achilies tang to come in..I have kole, naso and had a mata tang the bully was the mata tang but he got to big so I had to get rid of him...
Dude you'll be fine. There could be some territorial bouts but shouldn't be anything unless you get a pissed off tang which I have had before that cut me lol.
Try and get them in relatively small. I would put the hippo first. Also make sure you feed veggies as it known to decrease aggression.
Also aquascape so that the hippo has a safety zone to wedge him or herself in for safety.