The only time I've ever seen any of my crabs attack snails was if the snail was allready dead, dieing, or if there weren't enough shells of all shapes and sizes to go around. Now I make sure to keep plenty of extra shells laying around in the corner of the tank to prevent fighting and murderous rage. Once I started keeping these extra shells, I haven't witnessed a single shell related homicide between any of my CUC.
I will however say that a lot of these websites that sell pre-selected cleanup crews and monthly/bi-monthly "refresher packs" are simply to move more product. I, like many others have bought these packs only to end up seeing the damn things starve, even in a well matured tank. When I would order, I would see a 50% die off over the next couple months, then the remaining crew would survive much, much, much longer due to there only being enough mouths to feed that my tank can support. From my experience, whatever these websites tell you that you need, half it. This may also be a reason I don't see my crabs attack snails, because there is enough food for everybody, minimal competition.