It can be done , but will depend on the nature of fish you have and fish to add , PB's can be quite aggressively territorial , as with yellows .
I started with a orange striped bristletooth followed by a purple tang who were then in for around 9 month before I decided to add more , I've since added a yellow and a PB . Each took 3-4 days to settle properly and hold their own before being accepted to swim around together .
Tangs to add must be in good health and be of suitable size , feeding well etc .
The advantage I had is that my tangs dont rule the tank , I have a bluethroat trigger that will only put up with so much darting about before putting all back in their place .