I'd second ellencraig's suggestion to pass on Pisa this time. All that really happens in Pisa is you look at the tower, and you say "...yup, it really does lean...". Just like in the pictures you've seen 1000s of times.

Your schedule is so tight and there is so much in both Rome and Florence.

Maybe get some guide books and start making lists of what you REALLY want to do in the different places, and then let those lists drive your time split?

Romelle