I have quite a number of MTS (I'm guessing 100 snails) and a couple small clams in a tank with a soil underlayer and sand upper. I just used Miracle Gro Organic Choice, but same idea I think. They must burrow as they are all generally invisible during the day, but either they stay in the sand or they move up slowly enough that they don't drag the mud with them.
I haven't seen any issues yet, nor does it appear the layers have mixed. Tank has been running for a little over 6 months. The sand is about 3/4" deep over most of the tank.