If you've already run the #14 you might as well keep it. Especially if the calculated load is as small as you say and will not be added on to anytime soon.

I bought a couple of rolls of #14 eight years ago, just to say I had some, but never opened them.

Since the requirement to use AFCI's $42 each) came out putting all you can on one circuit is very important.

With the exception of Cutler-Hammer type CL, the breaker you use has to be the same as the panel you installed. As far as I can see AFCI's are all the same.

Good Luck from Columbiana, Alabama
Maurice Turgeon,http://thesemi-retiredelectrician.com