Unfortunately what shops fail to tell people is that fidler crabs need brackish water and must have a area of land on which to get out of the water,without it they do just die.Redclaw crabs canlive in FW but again need a area to get out of the water,they will also eat small or slow moving fish in the tank so are not ideal for community set ups.
There are many kinds of shrimp that can be added to community set ups providing that there is no large/aggressive or or nippy fish in the tank.A word of warning though,some of these will take small fish.
FW "lobsters" or crayfish are a real no no ,these are extreemly profficiant predators that will soon empty a community tank,they can inflict serious wounds on larger fish too.
