I've never had assassin snails, so I don't speak from experience. But I did a quick Google search for you, and I don't think you should worry too much about them eating each other if that's your worry? They mostly go after different snail species or incredibly sick/dying fish. Though a few sites did say that they will resort to cannibalism if they're starving.
I'm not entirely sure though, so hopefully someone who has actual experience will come along! (You might already know all this too, if so, sorry!)

 

Do they have anything to eat? Unlike other snails they do not eat plant matter so if there are no pest snails for them to eat you may want to return them.

 

I actually have a assassin snail name Thanos, who I had bought to take care of my Malaysian trumpet snail population that came from some hitchhikers on some plants (oops). I was doing research on assassin snails because I needed to know how many I needed and they usually got along with another as long as there was enough food.

 

Yep I have the Fluval bug bites and put in a bit more than Kas will eat for the shrimp and now snails

 

Ive had 2 assassins for going on 2 years (and a zebra nerite). They wont eat each other, and in my experience haven't bother the larger snail.

They do eat the pest snails that come with plants (MTS and "pond snails"), but not as voraciously as i'd hoped. If there are no snails to eat they eat left over fish food like any other bottom feeder. I use sinking carnivore pellets for my loaches and I assume that gives the assassins the protein they crave.

 

Nice. Thank you.