Assassin Snails are freshwater members of the Whelk family. With Whelks some are,some aren't, poisonous...most all are predatory...some sting their prey,some just overpower it.
Considering that an indepth study has never been done on Clea helena (most common "Assassin snail" in hobby),I'd say...
if in doubt,don't do it.
Not worth the risk to be the one to find out they are.
Just think about it.