The Trochus snails will be fine in a bare bottom tank. They live on hard substrates without sand, anyway. Most of the Nassarius species that you see in the hobby spend the majority of their time buried in the sand or rubble, and may or may not do as well in a tank like yours. Burrowing in the sand may be primarily for defense, though, so if you don't have anything in the tank that will bother them, maybe they'll do OK without sand?
Cheers,
Don