Rome to Florence via train

If you have a return flight on the same ticket, not boarding the last leg from Rome to Florence will void your return. And even if you have separate tickets, airlines have started to give penalties to passengers who do not complete their part of the contract by dropping out halfway into their purchased itinerary. I've seen such fine print on Air France lately.
So if you do decide to take the train, which is a good idea, you need to rebook your flight accordingly.

Otherwise I think that there is one daily train, maybe two, straight from Rome airport to Florence. Your best bet would be to take the Leonardo Express to Roma Termini, which runs every 15 minutes, and change there for one of the frequent Italo or Trenitalia trains to Firenze Santa Maria Novella, abbreviation Firenze SMN. A tram then links SMN to the Florence airport.

This is not worth the hassle. First there is no train from airport to airport. I think (without checking schedules) that there are two direct trains a day from FCO to Florence that are direct -- no change of train. Personally don't think you would save that much time. There is at least one train an hour from Termini (Rome) to Florence. Have a nice breakfast, catch nap, and stay with original schedule.

I would change the whole thing (you need to modify your reservations to avoid penalties and/or make your ticket void).

Change your trip to Rome. Take the train to Florence, and pick up the rental at a city office, chances are you will save.

Also depending on where you are going with your car, it might be beneficial to pick up the car in Rome. For example if you are going to Montepulciano area, it makes no sense to rent in Florence. Rent in Rome or Chiusi.

Once you consider the train and transfer times, traveling from Rome airport to Florence airport by train + tramway or taxi, will take approximately 4+ hours from the moment you walk out of the baggage claim in Rome to the moment you are at the Florence airport car rental center.

Without having to change your ticket - consider getting a day use room at FCO take a nap and a shower keep your flight to Florence.

Definitely food for thought. By the looks of it I will probably simply have to enjoy my 7 hour layover in Rome. It could be worse! Thanks to you all for your advice. Really appreciated! Ciao!

You can either be in downtown Florence at 10:46 AM by train or arrive at Peretola Airport at 1:30 PM by plane. The transfer at Termini consists in a 200 meters level walk without leaving the gated area.

Get the car in Florence in Borgo Ognissanti road and save both time and money. I am positive you can drive straight out of Florence without going back towards the ZTL.

Thanks for the info. More for me to consider!