Mine never bothered my skunk, but the shrimp was there first. I've never observed mine going after any snails or crabs, but I have noticed that my CUC doesn't seem to last long. Wouldn't hold up in court, but...
I had some flat worms when I first added him and he made quick work of them.
When you QT him, have a container filled with sand big enough for him to bury himself at night as well as a place to hide the first couple of days.