not for long
If it has gills and it can swim.
no
fire belly toad.
Ten gallons for two or three is fine.
Yes, hermit crabs don't usually fight. You could give the new hermit crab a bath so that it smells the same as the other hermit crab in you tank.
Goldfish should only live with goldfish with the exception of weather (dojo) loaches, bristlenose plecos, or mystery/apple snails.
Bettas do not mind snails at all. A crab with pinchers would dammage the bettas fins
A tank
Well for the safety of your hermit crabs i would use the basic rule of 1gallon of tank space per hermit crab..... so you could get any size you want.... but lets say you had a 10gallon tank so you could have 10 hermit crabs in the same tank.
It can be 3.5 ft deep, but you may prefer to give them more "land" space.
You can keep a horseshoe crab in a saltwater tank, as long as you have a large sandy bottom. If the horseshoe crab becomes too large, or starts knocking things over in the tank, get release it back into the ocean or get a bigger tank.
-( 0 0 )- -| |- -(____)- <<<<< this is an example of your crab!