It can be easy to get confused with so many statements being made. To avoid confusion:
* If you have 14 gauge wire it must have a 15 amp breaker
* If you have 12 gauge wire it must have a 20 amp breaker. No, it's wrong to put a 15 amp breaker on 12 gauge wire. Yes, it's technically safe as the wire is "overbuilt", but it can cause confusion and problems in the future. Just do it right.
* a 12 gauge circuit protected by a 20 amp breaker should ideally have 20 amp receptacles. It MUST have a 20 amp receptacle if only one is being installed. As shown above, you MAY use 15 amp receptacles if there are multiple.
@Charles Cook
So bottom line is your garage with a 12Amp breaker, 12/2 wire and mixed 20 and 15 gauge receptacles is fine.