Gary,
There's no accurate way of telling which Binario (Track) No. your trains will arrive on, especially as these are sometimes changed at the last minute. Your arrival platform doesn't really matter. In order to find the correct departure platform, I'd suggest checking the electronic board as soon as you arrive, looking for the number R-3056.
I assume you're planning to buy tickets for both trains in Napoli? As your second train is a Regionale, DON'T forget to validate your tickets prior to boarding the train on the day of travel (tickets for the Freccia train don't have to be validated, since they're specific to one train, date and departure time). DO NOT validate the Regionale tickets in Napoli, as their validity may have expired before you get to Florence.
Twenty minutes is MORE than enough time for a easy change like that. You may find it helpful to have a look at THIS short video which shows the layout of Firenze SMN.
Regarding your second question, you could consider using a departure from Montecatini Centro at 05:33, arriving Firenze SMN at 06:37 (cost €5.20 PP). From the station, use a Taxi to the airport as it's a relatively short trip and the Taxi bills at a flat rate (about €28 as I recall, although there may be a surcharge at that time of the morning). If you don't want to get up that early, your next best bet would be to stay in Florence the night prior to your flight. Ask your hotel to pre-arrange a Taxi to the airport.
Happy travels!