One bedroom of the house had a short because of the insulation flaking off. I'm rewiring the 15 amp circuit that runs the lights/outlets upstairs. I ran #12 to 1 bedroom's outlets/lights and would like to tap into the old wiring from the rest of that circuit; that wiring is #14. Is there any problem having that room wired with #12 at the end of a run of old #14 wire? The alternative is to wait until I've completed the whole circuit including the main wire to the panel, all with #12. Thanks! This site has given me a lot of info

Just neeeds to remain on a 15 Amp breaker till all #14 is gone.

Thanks for the answer. I had a follow up if you don't mind. How can I ground that piece of the ciruit that I've done now with cable? Ie/ How can I ground it to the old conduit in the circuit so that the wife doesn't electricute herself? Eventually I'll ground the whole ciruit to the box once I've done the whole circuit with cable. Thanks