Hermit crabs will die. They are not a fishtank crab. They need land, not a total water submersion.
Fiddler crabs can live in your tank, but.......................
They will find a way out. Either up an airline, the filter tube, the silicone in the corners, anything that sticks into the water they can climb. I had them in every type of tank, from oscars to neons. "Most" fish can/will get eaten if cought by the crab, though a bigger fish will not.
They are GREAT bottem feeders and cleaners. They will add to a tank with a variety of fish, and they look and move cool 8)
But keep in mind, they are not just scavengers, they are predators. I have had good success with large gouromies, bala's, angels, and some afr. cichlids.
You can get stuff to "enclose" the top of your tank so they cant get out. And If you do get some, buy 1 or 2 females first, the ones with both small claws. They will do far less damage to any other fish than the males with 1 large claw. And you'll be able to tell if they can get out or not.
I had quite a few live in my Whisper 3 filter. They would climb up the intake and just chill out in the filter. They actually helped my filter packs to last longer!