If you have ich in your tank, IMO, every fish will get it. Their healthy immune systems are no match for the sheer number of ich bugs that a single cyst produces. Either a tank has ich or it doesn't. It won't just lie in wait for a new fish. The bug has to attach to a fish or perish; if there hasn't been any sign of ich; it must be introduced...thus QT.
Bottom line is that while some fish may be more prone to ich (this is said of many tangs, I just don't know that for sure); any fish can get ich, and in a confined system--most will get it if its introduced. If you QT every fish; ich is easy to cure, even on tangs. If you don't; any fish can introduce ich to your system and you'll face a problem many times the value & time invested in a QT. A QT is VERY cheap insurance.