I buy shrimp and crayfish (when I can find it) from the grocery store and feed it to my FH (and other fish). I'd worry about parasites also and that alone would be reason enough that I wouldn't do it.
i think it's safe to feed both to your FH. as for the parasites i'd be more concerned about the clams then the prawns. also when feeding your fish these types of foods. be sure you get unsalted. alot of companies offer them both ways (salted/unsalted).