Ich won't affect snails, so no worries there. But has the tank finished its cycle yet? In that other thread, it sounded like you might've put the damsels in a little earlier and you were still cycling. If you have any ammonia or nitrites present, don't put anything else in there until it's done cycling.
Regarding the fish... not really sure what to say since there's no way to know what exactly you saw. I *do* know that a fish with ich can look "salted" one day and clean the next. I've seen it in fish stores, and experienced it just recently with a wrasse I'm treating. It was in the QT for a couple days when it showed obvious signs of ich. I immediately started dropping the salinity and the next day the spots were gone. About 4 days later, a couple spots reappeared. Since then, it's been clean.
So yeah... you could've misdiagnosed, or you could just be seeing the normal life cycle.
