Is it ever acceptable to attach 14 gauge to 12 gauge wire? I have a remodel situation where I have a 20 amp/12 gauge wired circuit powering outlets only. I want to tap off the end-of-run outlet to power an electric-venting skylight. The skylight comes with 14/2 wire. Can I tap off the 20 amp circuit to run the skylight, which would require connecting 12 gauge and 14 gauge wire? I thought it might be okay since the skylight motor only draws 60W max and would never pull more anywhere close to 15 amps through the 14 gauge wire, but I still feel uncomfortable doing this.

You would only be completely safe if you put a 15 amp fuse on that circuit. You can't control what people might plug in to those other outlets on the same circuit. The power runs all the way to the end of the branch whether that appliance is turned on or not.

When you say the skylight "comes with" 14/2, what do you mean? Is this part of the window assembly? Is it hard wired?

The skylight is pre-wired with a 30' length of 14/2 extending from the motor. So, it is hard wired and came that way.

Then that is not branch circuit wiring and can be wired from a 20A circuit. This is the same theory that a light fixture hard wired with #18 wire can also be installed on a 20A circuit.