Terry, you might have missed this but msjinkzd's golfball sized mystery snail was ganged up by her assassin snails a few months ago even though the assassins had the option to eat other snails such as ramshorns. Mine also work in groups to take down canas larger than themselves otherwise the canas can simply retaliate since the canas themselves are aggressive. If the OP has only assassin snail, it'll probably work but beyond that, I don't like to risk the other snails meant as pets with the assassins. We can't underestimate the assassins. They are meant to kill as they live.i keep adult apple snails with assasins, but wouldnt chance it with any thing smaller.
Snails don't carry diseases that will harm the fish.