I have an inch of sand and an inch of dirt under it, but I was afraid that the ten dwarf cories I'm contemplating would disturb the dirt...
+1My panda corys never really "dug" around. They did do face-plants in the sand when they were looking for food. I have heard that Plecos are very destructive in a planted tank, as they grow bigger. I don't think your dwarf corys will be a problem.