Diffusa (what you'll almost certainly be getting) won't harm your plants. I wouldn't put them in the breeding tank, since apple snails need a minimum of 2.5 gallons each - they have a fairly heavy bioload. I've got my baby apple snails and my betta fry in the same growout tub with no problems. I even have some baby Pomacea insularum, which are much larger as adults (closely related to canas) in there. Thetend to nom on the plants a bit, but don't bother the fry.
How warm is the discus tank? I wouldn't put snails in a tank heated to over 83*F. It speeds up their metabolism way too much - they eat more, poop more, and die much sooner.