Once again, this is hindsight and personal preference on my part. CT is worth visiting, but so is everything else. If CT is a must-see on your list, your latest itinerary is certainly doable.
Rome has so many attactions. If we were not travelling with a group of friends who wanted to visit the other cities, i would have been inclined to just set up in Rome for the whole 10 days to see more and to relax more. With our 10 day itinerary, we still felt a bit rushed.
And we had some regret that we did not spend more time in Florence on our trip. Two nights only gives you one full day. My wife and i stayed in the city and did some of the major attractions, Duomo, Uffizzi, and Ponte Vecchio (squeezed in the Academia on evening of arrival, rush rush rush), but there were a lot of other things in Florence which we could not fit in. Our friends opted to do a guided bus day tour of Siena and San Gia...(?) and did not see much of Florence. We could do one or the other but not both. It is difficult to fit it all in when time is limited. Florence is in the heart of Tuscany and would have been a great base to explore if we had the time. I even prefer Florence over Venice, but I would not recommend cutting a day from your first stop. You really need that time to overcome jet lag and start your honeymoon in a relaxed enjoyable way.
Adding cities to an itinerary adds travel time and the converse is true. A 1-2 hour train ride really wipes out close to half a day and a 4-6 hour train ride can easily wipe out a full day when you factor in packing/unpacking, checking out/in hotel, getting to and from train stations and getting something to eat.
I am a big fan of driving and wished we had rented a vehicle for part of the trip just to try it in Italy. I looked into it and it is not too expensive to rent. Yes most cars are really small in Italy because cars are taxed every year based upon engine size. In the end, we opted to save time with our group of six and just took the highspeed trains. You can easily take a bus or train to Pisa/Lucca or Siena or San G. from Florence.
Keep in mind, there are all sorts of potential traps for dumb tourist drivers (driving in certain city zones, parking, speeding tickets and adapting to the local aggressive driving habits). We hired a private driver for shore excursions to CT and the Amalfi Coast and saw the narrow twisty mountain roads. It would have been fun in my Miata, but I am sure I would have scared the crap out of my wife who was already nervous even with our experienced driver.
If you do drive, be aware that Italy uses an average speed method of catching speeders. They have cameras everywhere taking photos of your plate and then calculating your average speed. They will ticket the car rental company and you will then be charged under your credit card. Big bucks from what I have read.
I envy you. Planning the trip is as much fun as going on the trip. Have fun.