I quarantine EVERYTHING. I've had a couple of bad experiences in the past with delicate fish that I was told couldn't be quarantined, so I didn't quarantine and wound up having to pull the whole tank apart and treat all of my fish. I know this doesn't work for everyone, but this is what I do, sometimes it takes planning ahead on my part, but it works for me. When I quarantine live rock grazers, I start in a QT that has been up and running for a month or more, with live rock in the QT and some algae growth. I usually try to be quarantining a small, hardy (read easy to get feeding on prepared foods), mellow fish at the same time because in my experience it seems to helps them settle in and start feeding sooner to not be in isolation.
If any disease is present I treat in a hospital tank after they've started eating what I offer and aren't only grazing on the LR. If they require live food, I harvest it out of my refugium and add it to the QT as needed. Since I don't medicate in the "established QT" I can have sand in there too if needed. Yes, it isn't the same as a fully established reef tank, but so far it has worked for me with every fish that I've tried from angels, to tangs, to gobies.
It's a pain, but I HATE tearing down my tank to treat every fish that I have, so I do everything that I can reasonably do to avoid that.