I don't think isopods eat snails but snails. I had some Cepaea nemoralis shells in an isopod tank for years and they didn't get eaten. Snails don't like to be disturbed much so too many isopods could kill them through stress.
Looks good. I can't tell on the moisture so you can just keep one end or corner pretty wet and let the other dry out so they can self-regulate. Yes, it often takes a good number to get them out in the open all the time.