Fairy wrasses can compete with mandarins, but the big ones to avoid are the lined wrasses (six-line, four-line, mystery wrasse, etc). They are direct food competitors and are much better/faster at it. Other than that there aren't too many fish that will compete for the mandarin's food source. I wouldn't worry about shrimp or crabs competeing with the mandarin for pods, though. They wouldn't have nearly as much impact as a fish would. I'm sure a mandarin would eat shrimp larvae, but again, they are such slow feeders that they probably wouldn't get to them before something else did.