I would just watch the yellow and hippo before moving them. Yellows can get pretty aggressive when they are the established tang but I had never had a problem putting hippos in with them. I'm not saying he won't pick on the hippo just that he may not.

I'm pretty sure your yellow would pick on a new sailfin since they are similar species so you probably should try to get a sailfin that is much larger than the yellow if you want to add one.