I've got mine in a 40litre with about 15 litres of water in it, it appears that the amount of water is fine to keep the crabs healthy (now 2 years old) with 50% water changes fortnightly but I feel that there isn't adequate space for them to walk around (approx 45cmx30cmx30cm). If I get a new tank for them it would be at least 70cm long. I fluctuate the salinity between SG1.008 and SG1.014 at 25degC
Apart from the fact that it's illegal I don't know if you can use beach sand - after you boiled it up good and proper it'd probably be ok for crabs but not fish.
Use sand or aquatic soil or a mixture of the 2 - fiddlers are from the same kind of area as mudskippers and so have the same requirements. They spend a lot of time out of the water so either a sand bank and/or plenty of rocks and logs protruding from the water is necessary.
IME mudskippers won't work though some say that the indian dwarf mudskipper might, I don't think that many fish will be compatible to be honest, if they're too big they'll scare or eat the crabs meaning that you won't be able to see that big claw waving at you or if they're small they might spook the fiddlers and be atacked by the claws.
