Which species or groups can you recommend for their hardiness?
Turbo fluctuosus is my go-to species for hardiness, adaptability in what they're willing to eat, and also safety from many CUC crustaceans like small to medium hermits. Many other species in the Turbo genus are similarly hardy and adaptable. I'm still puzzled by Andy's post with regards to those, since T. fluctuosus has seemed to me to be about as bomb-proof as snails can get and will also eagerly eat nori and other dried seaweeds if food supplies in the tank run low.
Some of the Trochiids are also good CUC members, but the names for the ones I've had that have shown the most CUC potential are escaping me right now. Cittarium pica is technically a Trochiid, but it gets absolutely HUGE, so probably not so good for a small tank. There is another striped species though in particular that doesn't have the bulldozing capacity of T. fluctuosus since it's a bit smaller, but it has seemed to be just as active and hardy. I'll have to dig up the ID I had for that one and get back to you. I don't think they're all that common unfortunately, although the availability may differ by region.