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I have had fiddler crabs for some time now and a lot of the info you have is great. I also have moon crabs and hermit crabs and before I became a mini pet shop due to my sons I had them all in one tank and they got along just fine. I have them all in different tanks now, they all have different attitudes. My fiddlers like to climb and swing craziest bleep I've seen and I've seen crazy things.
I'd get rid of the fish pebbles get sand, and have fresh water and something with salt water for them because they are salt water creatures. A 3 gallon tank is a bit small for 6 but the pebbles will break their legs. Good luck