Connecting 12ga wire to 14ga that is connected to 15 amp breaker
Yes as long as the breaker is sized for the smallest wire in the circuit you can use a larger gauge and it will even pass inspection. Many of us frown on using smaller wire when the connection in the panel is a larger wire because some one may up size the breaker seeing #12 in the panel not realising there is #14 in the branch some place down the line. But 14 in the panel on a 15 amp breaker going to additional devices wired with #12 is legal. I have done this on lighting circuits when I ran out of 14.