Looking good!
Japanese firebellies are tiny, and they can only eat the smallest and laziest fish. Red-spotted newts are also pretty small. You can keep these guys with any of the small shrimp species; you might lose a few juveniles, but they won't eat the adults. White cloud mountain minnows, or any of the small 'hitchhikers' you see in feeder shrimp tanks (bluefin killifish, golden topminnows, swamp darters, etc.) would be suitable fish. Dwarf crayfish (Cambarellus species) would work, but any other crays are too big and destructive; they would kill their tankmates and likely demolish your hardscaping too.
I don't know much about brackish species, but crabs are definitely harder to keep fish with than newts are.
There are no semi-aquatic snakes small enough to be comfortable in that setup.