You simply cannot use 14ga wire on a 20 amp circuit.

Any electical inspector that sees that in a box will simply
flag it and make you change the line or the breaker.
(just as an FYI)

If you pull too much near the 15amp side and exceed
the limits it wont stop for another 5amps. In effect you
can be overloading the line. It may not happen on your
lighting idea, but why bother with the risk for a few bucks
more on getting the correct wire.

Using 12ga on a 15amp is perfectly fine. Its nice that down
the road if you need to upgrade you can just change
the outlets and breaker and you got a 20amp line.

Tom