Nassarius aren’t super predators. I have several species that can’t right themselves in there with mine and generally I catch the ones that have fallen before they’re able to eat them. Spiny nerite snails, cerith snails, and margharita snails all coexist fine with my nassarius.
I agree, they are not out to only go after snails, but they are super opportunistic.
The ones in the picture aren’t. Whelks have a tattoed trunk and a trap door that closes when they retreat into their shell that you can see on the end of their bodies when they are moving. Whelks also have a sort of different shell style. Or at least mud whelks do. Hard to explain without having one in person for me to take a picture of.
Why specifically spiny astrea? I've never actually kept those-- only the standard astrea.
Spiny astrea are natural tanks as well. They have a door and their shape means it’s really hard for them to fall over in such a way that they won’t be able to right themselves.
