Bannerfish are no more susceptible to ich than other fish that are being harassed. The problem was the Sailfin picking on him and not a compatibility issue. They ARE compatible and would have been a better mix had the Sailfin gone in the tank after the Bannerfish. You will most likely have the same issue with any new fish you put into the tank that has any size to it. It's a threat to the Sailfin so it needs to show "the new guy" who is boss of the tank.
As for the spots, most likely it is ich. It may have come with the new fish or, because ich has multiple stages, may have been in your tank all along and just needed a stressed fish to be present to attack it. I strongly suggest you remove the new fish to a hospital tank and treat for the parasites and make sure your main tank does not come down with an infestation. Afterwards, after all is healed and confirmed safe, I'd remove the sailfin and reintroduce the bannerfish and let that fish get settled before reintroducing the Sailfin. Better yet, decide what other fish you want in the tank and maybe bring the Sailfin back to your LFS
until you finish your collection then add the Tang last. There will be more harmony in the tank this way. I also suggest you set up a quarantine tank that you can house any new fish in for a minimum of 6-8 weeks. Most diseases will either have attacked the fish or shown up within that time frame so you can better treat them in a smaller tank. The new fish will also have the chance to get used to your feeding regimen and foods you feed without the harassment of the other tank mates eating it all first.
Hope this helps