Can different gauge wiring be used in the same circuit?
The OP has a great reason for using 12 ga. wire.  He already has some on hand, so using it is cheaper than going to get new 14 ga. wire.  It's perfectly safe to use it.  Don't worry about confusing future workers.  If they want to add something to the circuit, they will have to go turn off the circuit breaker first, and then they will see they are working with a 15 amp circuit.   Then, if they have any brains / experience at all, they will see that there is 14 gauge wire hooked up to that 15 amp circuit breaker, so they will know not to upgrade it to a 20 amp breaker.  Your plan is perfectly fine.