You are veteran hikers, so as long as you have appropriate gear for Winter, you'll be fine. The trails from Lucca to Siena will most likely be wet and that Tuscan mud can be sticky, but the route is generally very visually rewarding, even in the heaviest rain (some of which I encountered on my ramble from Lucca to Rome in late April to mid-May this year). I booked ahead only one or two days and occasionally on the same day. San Miniato had a limited number of rooms for pilgrims due to a conference that occurred on the same day I passed through and San Gimignano was overrun by tour bus groups, but I found a room right on the main square (with two! gelato shops nearby). No other housing issues, however, I can't say if all the ostelli will be open in February.
The VF book by Sandy Brown, the list of accommodations from the Confraternity of Pilgrims to Rome and Booking.com are good sources, even with some overlap.
I encountered a few areas where there were flood warnings and creeks to be crossed on rocks or logs. Only near Colle di Val d'Elsa (a beautiful area between San Gimignano and Abbadia a Isola) was the river running too high to make a safe crossing on the rocks, so I walked along a busy boulevard and a short stretch of highway to bypass that section of the trail. Just keep an eye on a weather app to see if there is flooding along your daily route.
Buen Camino.
