I had a Blue, Purple and PBT in my 120g for a while no prob, moved them to a 170 no prob until my PBT died a month or so later.
I put my purple and blue in the same day and never see them fight. My purple is pretty aggressive to new tank mate though. I like blond naso's and browns too...
As Naesco said PBT's are harder to keep and do tend to die for no reason (huge ich magnets), but they are not impossible. As well they are fairly active and aggressive to other tangs.