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
** Collect native species only so if they over populate you can release the over flow outside. Non native species should never be released outside. In most places it is illegal to do so.