Don't mix the two in the same tank, crayfish are very good at getting at and into most snails to eat them and snails can readily pass on parasites and diseases to the crayfish after they've been eaten (some of which can become fatal). However, if you have a tank with very peaceful, small fish you could try some of the dwarf species that stay under 3", most of these are peaceful in nature and less carnivorous so they won't see the snails as a food source. And they come in a variety of colors and patterns from natural colors to wild, bright colors.