Ciao Margaret!
If you are taking a day trip to San Gimignano you may decide to join a guided group tour by bus or reach San Gimignano on own by SITA bus departing close to the Santa Maria Novella Train station:
From Florence to San Gimignano you will have to change in Poggibonsi.
The Bus company that organized the sevice Florence-Siena is called SITA
Tel.055 48365
http://www.sitabus.it/
If you wish to stay in San Gimignano for a night, you will have to catch another bus to Siena the following morning and then another one from Siena to Rome. I am attaching you the bus company website:
http://www.sena.it/Autobus/Roma/Siena/219-234/49-1.html
I would honestly avoid travelling by bus and having to change several times along the way...if you are travelling with luggage it can be very hard and painful mostly in Siena where the bus companies have different departures.
I would honestly get back to Florence and take an easy train connection into Rome.
Have a wonderful stay! A presto, elena
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