I don't have the experience that Dougall does and I keep only hardy, relatively easy species of plants, but I always use 1.5" of soil capped by about 1" of sand. I have very little travel between sand and soil and no problem with snails burrowing into the soil layer that I've noticed.Crypts do.. the rest are stems, you should just need to bury them enough.
I would do 1-2" of sand I think.. but it's not an exact science.
WYite