Not sure about how close the airport is for these cities but, instead of a tax,i you may be better off with a train or metro.
At his time of year the days will be short and you will have to make the most of the morning / evening sun. I would suggest travel between cities be mid day, early am (before sunrise) or late evening (after sunset). Both Sienna and Oriveto are small enough that you could photograph them in two full days with the evening the day you arrive and the morning of the day you leave. Both Rome and Florence have more than enough sites to occupy the balance of your time.
Florence is small so you can do most of it by foot. You want to stay in the historical area so you can step out the door and be there. I climbed the Bell Tower instead of the Cathedral Dome because I wanted the dome in my photographs.
Rome is quite large and unless you have zeroed in on a specific area to photograph you probably would want to stay near a metro stop. You should check out the times the monuments / building are lit at that time of year. Photographing them just after sunset gives you some light in the sky to balance the brightly lit buildings. If you are interested in photographing inside the Vatican Museum, and are in Rome on a Friday night, check to see if evening tours are available in February (much smaller crowds). Do not worry about the Sistine Chapel as no photography is allowed there (they are VERY serious about this).
The above is based on a 30 day trip I took to Italy two years ago.
