they will absolutely try and attack the diffusa. The nerites are a little more safe as the angle leading to their body is more difficult for the assassins to access. That being said, I don't recommend keepign any snails you want to keep alive with assassins.
Yup, what Rachel said. I had no idea that my assassins would eat larger snails until one of my other vendor friends told me that they occasionally feed their assassins things like giant ramshorns and Canas (giant apple snails). Apparently they will eat any other snail they can actually get to, so it's not a good idea to keep any non pests with them.