Happy Clam,
If you are still out there, I would not worry about the ICH so much. I have had issues with it on multiple occassions, meaning I was not smart enough to research it as Freed suggested. As Chris said it can be introduced in several different ways perhaps just not with fish. If your tank has been up and running several months with no signs of ICH, you are perhaps good to go (they need a host to live). As I said I have had issues with it mulitple times, I did begin the QT practice but it had already been introduced into my tank. Make sure you QT and do not let it in to start with. I read somewhere that it can remain with a host up to 11-12 months (due to max. cell divsion I think). I know I transferred a Hippo tang from one tank to another and it broke out. I do not know if it was introduced many months previously and was living in small quantities or if it came in on rock or coral. Either way, it was in there an it was shocker to me.
Since the PBT will soon be the boss of the tank, the stress should not be so bad. If there is ICH already in the tank it must be in small quantities so the PBT may never come across it. If it were to get it, sometimes they fight it off on their own. Just QT the tang, make sure it eats well to build up it's natural immune system, perhaps introduce it after the lights are off to reduce the stress. It should be OK - just QT it from the start and do not panic if you see a couple of spots.
I have had mixed experiences with cleaner shrimp. I have seen them set up a cleaning station. I think they will help if there is a small outbreak in which the fish can probably beat. Same for cleaner wrasse. Most do not live very long but now some are available from Africa (I think) anyway they clean well and will eat flakes.
Good luck