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.
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.
WYite