They are reef safe, but make sure you're getting Peppermint shrimp and not Camelback shrimp, which can look similar if you don't know any better. Camels will eat coral. It is completely hit or miss on whether or not Peppermints will eat Aiptasia, if you add 5, you may get 2 that actually eat Aiptasia. I only have 2 in my 155G and neither eat aiptasia. They will steal food from corals when you're trying to spot feed, so I normally just give them a big chunk of mysis to keep them occupied.
A word of caution: If you plan on keeping a mandarin, you may want to re-think Peppermint shrimp. Peppermints eat copepods and between a couple peppermints and a mandarin, the supply can deplete very fast. I'm always afraid that my mandarin might not be eating enough so I seed my tank with pods every week.