Yeah they are borderline size, the orientalis didnt even bother about the apple snails I had with them for a while, but they eat ramshorn snails like there is no tomorrow. The male zebra snails are about half the size of the females and I suspect they could eat them if they could get them out... I know the fish in the pet store ate them as there was lots of empty shells, what a waste of great snails eh...
On the other hand the ramshorn do not have an operculum, the zebras do. It would be great to hear from someone who have kept newts together with zebras...
I know axolotls can eat both zebras and apples...
Also when my orientalis eat ramshorn they somehow manage to pry them out of their shells, because the shells are left behind...