From what I understand, they only really tend to eat other snails which are not of the same species, so you need to ensure you can provide enough "food" for them. They aren't so much "Clean-up crew" like other snail types
You don't have to have other snails for them to do well, they will eat lots of stuff just like regular snails do. So you can get them but be sure to not get a ton as there might be a restricted food source in your tank
it is worth getting them however they rarely eradicate snail infestations as the prefer to eat older snails leaving the babies time to breed before they are big enough to turn into prey. assisins eat all sorts besides snails so you shoildnt worry about them starving. with a bit of look you might even get them to breed. i think they prefer soft warm water with a sandy substrate to entice them to breed