Good plan, we visited Florence and Rome before our cruise last October. We just did the opposite because we went from Venice to Florence to Rome. The train is super easy.
We stayed at Hotel Damaso in Rome, very nice hotel and excellent location. We had them hold our extra luggage while we were on the cruise and then went back for a night before our flight the next day. I'm sure most hotels would hold your luggage while you go to Florence. We used Stefano's Rome Cabs for transfers to and from the cruise port and also to the airport. They were fantastic!!