Personally I think putting more than one aggressive tang in a tank that size will be a crap shoot. I know some people get away with it, but I think the probability is high that you'll have problems, if not initially, in the long run (or both).
Myself, I've always avoided PBTs altogether given what I have seen of their survival rate in captivity, but again, that's JMO.
I only have two tangs in my 6' long 180 - a Kole and a Mimic (both species are mild mannered as tangs go) and after two years of peace and harmony (and quite a bit of growth) they recently decided they don't like each other anymore and are now routinely beating the crap out of each other...