I'm wiring 2 spare rooms in my attic and I have some short 12/2 cables lying around and I'm wondering if I could run a 14/2 on a 15 amp breaker from my panel & use the left over 12/2 cable to connect the receptacles. Will this be illegal? Any help will be greatly appreciated.

With a 15 amp breaker you should use 14 gauge wire OR LARGER. You should be fine using your left over 12/2