I would consider NOT getting any more fish. the Blue Hippo tang is pushing it for that tank and if it is already of a decent size takes up a lot of bioload.. If possible I would suggest giving away the hippo and going with a smaller species of tang (Bristletooth family is great). You are going to waste a lot of money to stressed out fish if you keep going about adding fish before adding them in. I think a 90 is pushing it for a yellow tang too for the latter part of their lives. Other things play into factor here too, like what type of skimmer you have running in your sump. It may say on the box its rated for the system but a lot of companies overstate this important detail.
Like I said, If possible.. Give away the hippo and stock either a smaller tang or smaller fish, the Bristletooth tangs are awesome and I recommend them. Do extensive research before you go to the LFS and get an idea of what you want and what could actually work even 5 years from now when the fish is full grown.
Another alternative since you already have in reality 6 damsels (2 clowns, 3 black/whites, 1 yellow blue) I would suggest, once again ridding the Hippo. Not getting any tang and stocking a decent amount more damsels in place. Theyre cheap and add a lot of color, the only issue is if you have more peaceful fish in the tank with them, but since you would have all damsels you wouldnt be forcing any fish to their death, although some damsels may get picked to death its their nature and besides that.. theyre cheap. A sixline wrasse would probably be able to hold their own against the damsels ( I have no experience with them) but they are all pretty much referred to as terrors.
Good luck.
