I have heard they are sensitive, my experience is that a good aquarist can keep them alive easy enough. I've had a few deaths, but out of the 10 I put in my tank, I think I still have most of them. Its impossible to get a true count though, they hide too well. Mine tend to come out at night, but I see them most often right after a water change. The trick is to acclimate them thoroughly and do not stress the tank too much. I have had more sensitive fish, I had a Farlowella Acus that died after the first 25% partial water change 2 weeks after I bought him. He was a great algae eater, but by far the most sensitive fish I've ever seen.
Oh and I would be surprised if any Otos survived a dosing from coppersafe, otos are invertabrate catfish and should never recieve a copper dosing.
As for the snails, be carefull what you put them with, my brother recently added swordtails to his tank and they kept nipping at his existing apple snail making him very unhappy. Eventually he had to put a separator in his tank so the snail would come out to play.