Transfer time at Florence SMN train station, Bologna to Sienna

That should be fine. Trains in Italy are generally on time. While the station is a larger one, it is not "airport huge". I can't specifically remember the track layout, but usually the fast trains come into one side of the station and regional trains use the tracks on the other side. The station is a dead end station, meaning the trains come to a stop at the end of the track, you simply walk to the front of the train and walk over to the new track, usually no stairs, and not a long distance. This might help: https://www.seat61.com/stations/florence-santa-maria-novella.htm

Of course, be aware that if you miss your train to Siena, since it is a regional train, you are allowed to take the next (or any) train to Siena that day on your ticket, the regional ticket is not train/time specific.

You will be likely arriving at one of the central tracks, 6 to 14 or so. Go immediately to the head of the train. Turn right at the end of the track. Usually Siena trains leave from low-number tracks, 1 to 5; only track 5 is full lenght, the other ones are shorter and leave from displaced platforms, you have to walk a little along track 5. Follow signs.
It takes five minutes, but remember of the displaced platforms or you will be wondering where the low number tracks are. If your incoming train is much delayed you may not get to outgoing one, but there will be another one, usually one hour later (8:10, 9:10, 10:10, 11:10 and so on).
There are underpasses that may make the walk a little shorter, but they may be confusing and, if memory serves me, now they are closed due to abusive behaviour.

Yes, 20 minutes is fine. All the tracks at SMN are laid out end to end so it is usually a matter of walking down a few tracks. I would download and use the Trenitalia app which will let you know which track you are needing to head to when you get off. SMN is small but very busy all the time.

https://www.seat61.com/stations/florence-santa-maria-novella.htm

The only weird thing at SMN to be aware of is that tracks 1-2 are slightly 'outside' the station and tracks 3 and 4 are set slightly down the platform for 5 and 6. Local regionale trains usually use 1-6 so just be aware and don't panic if your train number is not on the main platform - just check the signage and follow the crowd because it's close.

https://www.amoitaly.com/img/deposit/deposit_map_firenze.gif

The only real issue would be if your train was late but if Trenitalia caused your problem they are required to make it up to you. Since the train to Siena is a regionale you could take the next one on the same ticket but check with the help window.

Have a great trip!
=Tod

If you get the Trenìt or Trenitalia app set up, you can track your train times, tracks and any delays in real time for a little extra information.
The train station in Siena (note spelling) is not right in the centre of town like most others in Italy are.
I believe there are frequent buses to get you up into the centre if that is where you are going.

Trains to Siena generally depart from track 5, however if you are taking a train with change at Empoli, then it could be also 1A, 1, 2,3, 4.
Your train from Bologna (assuming it's the FR 9301) should arrive at track 12. In the worst of cases (for example if you have to walk to no. 3 or 4, which are displaced platforms), you might have to walk no more than 300 m. (1000 ft). As mentioned above, if you miss the train to Siena, you can take the next available regional train.