Hey Sabrina!
I noticed in another thread you were planning to rent a car to drive to Tuscany. Is that still the plan or have you changed your mind and are planning to take public transportation? Have you been to Europe before?
If you are planning to rent a car in Rome just to drive to Florence, I suggest you rethink your plan. I think you'd be better off taking the train and then take a tour of the Tuscan countryside. To me, a car makes sense only if you plan to stay in an agriturismo and stay more than a couple of days.
Have you arranged all your accommodations? Mid-September is only a few weeks away. I guess you can just show up, but availability may be a problem.
How long is your entire trip? If you are moving around as much as you indicate, I suggest you seriously rethink your luggage plans. We did take one checked bag each on our trip to Tuscany, but we spent the entire time in one location. When we went to Scotland for 10 days, we were moving every other night. We only took two carryon bags each and did fine. My wife spent a week in Tanzania in the Spring. She took a checked bag full of medical supplies, but all her personal items were in two carryon bags. We are going to London and Paris in the Fall and we will only take carryon luggage. The trip is much more pleasurable without having to haul heavy bags all over creation.