I think you can safely add a few more fish, small ones though. A royal gramma would be a nice addition, if you have a lot of places for it to hide. Also a blenny or goby would be cool. Something else simpe would be a cardinal or something. I wouldn't add an angel or butterfly, especially if you plan on having corals. You could try a clam, but only if you have the right light.
As for the hippo tang, my buddy has a hippo in a 90 reef and it seems to be fine. Of course this depends on the size. A moderately sized hippo is going to outgrow that tank fast. I agree with the above post. You may want to explore returning it to the lfs or trading it to someone with a larger setup. It's a tough decision, but that would free up the bioload too, allowing you to possibly add another small fish or two. As long as you have a good setup, I think you should be fine.