Fiddler crabs or green spotted puffer and tank mates?

NO! fiddler crabs have to be able to get OUT of the water or they will eventually die..they are also kind of destructive (they dig up live plants) and they WILL kill slow moving fish! Puffer fish are also very agressive, so I don't think guppy or molly would make good tank mates with them. Puffer's mollies, and fiddler crabs are all brackish water critters too, so make sure you have the right stuff to care for them!