I've never heard of a reason to QT snails - but if you have heard something different, better safe than sorry. I've never had any issues with snails bringing disease into my tanks.
Tiger nirite snails are very pretty, and you can have just 1 in your tank. With large snails though you do need to make sure that you feed them enough, if you rely solely on the algae your tank is producing they will probably die.
Cories won't have any interest in algae pellets (well, they may be interested for a few seconds, but then they'll realize it's not something they want to eat). They aren't really algae eaters, and as long as you are feeding them bottom-feeder pellets and they aren't starving you shouldn't have a problem with them eating the wafers.