I currently have 5 in my 180. Lavender, blue hippo, purple, Thompsons and bristletooth. The lavender is the alpha of the group and would probably seriously injure or kill any other tang additions. So unless I have an unexpected death, I am done with tangs. In my experience, the blue tangs are more timid than most. So you most likely would not have an issue putting another tang in with that one. But if you get into the zebrasoma tangs, they can be aggressive, and it would be advisable to add them all at the same time. So in your case, you could easily add a bristletooth (which are also very timid), or acanthurus (such as a powder brown), but I would save the zebrasomas (yellow, purple, gem, etc..) for when you move to the 180. Then you can buy the new fish, quarantine it, and release them all into the 180 at the same time, before any one fish can establish territory.
There are other tricks people have used with some success. Putting a mirror on the glass at the same time you put in a new tang, might trick your established tang into fighting itself. I have also printed out a picture of a big yellow tang and taped it to the side of my glass. That didn't work particularly well, but I have heard of people using that trick with success too. But like I said, in your case, the blues are pretty timid, so it is unlikely you will have problems introducing whatever you want. I would just think ahead about the timing going into the 180 and shy away from the more aggressive species for now.
I wish you luck. Both the hippo and powder brown are up there as one of the most susceptible for ich. Be patient with quarantine, if you go with powder brown.
