Check out the flights first. If you do not care about direct flights nor care about the price then flying directly into Florence may make good sense. If you must see the AC then do consider flights into Naples. We are doing that in May ourselves. As Jean says, getting to Positano will take time from Florence, and you do not have much time.

And since you're so enthusiastic about Pompeii, you may want to go also to Herculaneum while you're there. And to the archeology museum in Naples, which has lots of the best stuff from Pompeii. All this it may affect where you'll stay. (Can one do day trips from Capri?)