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Everything would be fine until the crayfish molted. If the cichlid was aggressive enough it could take this opportunity to eat the cray fish. I have not had this happen to me personally but I have read about it because I wanted to put a blue lobster in my tank.
That and when fish sleep is when the crayfish will attack. This is all dependent on type but personally I don't trust big predatory inverts with fish and cichlids could harass the cray during the day.