Yeah, Apple snails and Plants definitely DO NOT go well together... i've had one demolish all the plants in my old tank once, and it was done within 12 hours...
I have a very large mystery snail in with my severums, jack dempsy, and salvini, and they all leave the snail alone. Occassionally a blood parrot will pick on it, but it can't do anything.
There have been stories about Cana Apple Snails killing long-bodied loaches. Haven't heard of them going after anything else though.
Cana's can eat fish. I've thrown in dead fish before and watched my Cana turn it into a skeleton very quickly. But when it comes to live healthy fish...well it'd have to catch the fish fish which is not likely with anything but slow moving, small, bottom-dwellers (probably why long-bodied loaches have been reported as targets).
As for whether the fish is a threat, well that depends on the fish really. But they are big tough snails so I'd figure anything that isn't big and aggressive or good at snail-killing would be fine.
an apple snail I had when I was younger would "stand" on the sides of the tank, and grab fish as they passed by. I woke up one morning to find it eating my ID shark, and the snail had munched on the tail half of it, and the poor thing was still alive.