it all depends on the demeanor of your YT and of your new additions. I've had some YT that were just ba$t@rds and others that could care less what else was in the tank with him.
The factors that needs to be taken into consideration:
How long have your YT been in the tank?
Are you able to remove the YT for a period of time while you allow your new additions to acclimatize?
What size PT,PB,SF are you planning to buy?
do you have a qt tank?
My opinion is IF all three fish are equal or slighly larger than the YT and if you put all three fish into the tank simultaneously, you should be fine although the potential for an ich outbreak would be very high. PT, PB and SF are all very tough fishes but depending on their health when you acquire them, they will need to acclimatize to the new surrounding and gain strength before the battle for dominancy commences. So I would suggest not buying them the day they arrive at the pet store after being in bags for 12-24 hours. If the LPS store agrees or if you have a QT tank, put all three fish into a tank and let them figure out their rank. Keep an eye on them for a few weeks and when they are all healthy & strong, throw them into the 180. If the YT is really mean, then the bullying will be shared by the three new fish until your PB gets into spastic mode and kicks the poop out of the YT
