The list of critters that can safely exist with hermit crabs is fairly short.
Isopods – beneficial tank cleaners
Springtails – beneficial tank cleaners
Food/soil mites – harmless
Fiddler Crabs/Halloween Crabs – aggressive diggers and are likely to eat molting hermit crabs.
Earth worms, beetles, centipedes, crickets, praying mantis, roaches: May stress each other, over populate, disrupt/harm/ kill/ eat molters. Crabs may harm /kill them. May carry/spread disease/parasites, especially with over population.
Centipedes – venomous
Millipedes – poisonous
Snails – Hermit crabs can kill snails
Frogs/Lizards – could harm each other, different habitat needs
Fish – inappropriate water for a fish