You really don't have enough time on the ground to go to all of these cities. Staying both in Florence and Siena would be redundant, as they're both in Tuscany. Stay in Florence and do a day-trip to Siena. Especially with this being November, shorter daylight hours, you'll have a better time in the "dark" in Florence's Piazzas. The fountains will be lit and beautiful! You could easily do 4 nights in Florence and finish the trip with Rome. Both of these cities have so many sites, and so much to do that you will barely scratch the surface, especially if you take into account the "day trips." From Rome you could spend a day at Ostia Antica, Ancient Roman Ruins, not as far as Pompei, but equally impressive, although smaller and much, much closer to Rome.
Rome and Siena have Many Visitors all year long, and I, personally, would feel better having a reservation in both places. I looked yesterday for a hotel reservation for my favorite hotel in Rome for January and it's BOOKED! Shucks.
I was in Venice a very long time ago in November, and maybe we were unlucky but it rained nonstop, which certainly diminished our enjoyment of the canals, etc. Much better in the Spring or earlier Fall.
Seriously, One night in Milan, 4 Florence (with a day trip to Siena (and Fiesole), 5 Rome, day-trip to Ostia Antica. Trust me, the non-day-trip days will fill up. Remember shorter hours of daylight will effect your outdoor hours in a smaller place like Siena. Get an early start on those day trips.