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a crayfish will probably eat the shrimp. I'm not sure about dwarf crays, I think they are ok but IDK. The cray really limits your stocking options, and cutting down the water volume to 1/4 limits it even more.
I don't even know how they are with snails. I have not kept the two together. I have kept red claw macro shrimp and I would move them into tanks that had a pest snail issue and they'd resolve it in a few weeks.
Hopefully someone with more experience with crays can chime in.