Siena to Rome train through Florence -enough time to change?

Consider taking the bus to Rome the trip is shorter than the train
https://www.flixbus.com/

Or just leave on the first leg a little earlier since it is Regional train to Florence from Siena meaning you don't need a reservation.

The track layout at Florence is pretty simple and all on one level, with no need to go up or down to a pedestrian tunnel or bridge. 24 minutes should be plenty of time, but still do try to get off quickly, rather than being the last one off the train. Exchanging a ticket at the last minute will often require standing in a line and there are deadlines to do so.

Platforms are frequently not posted on the departure board/monitors until 15 minutes before, so you have plenty of time. The bus takes a little under 3 hours, leaves from the center of town; train station a little ways out.

You can walk from the first to the last track at Firenze SM Novella in 10 minutes.

If you miss a connection because of Trenitalia you are always protected, exchangeable ticket or not. So you could have saved some money buying a discounted ticket.

A different train stops in smaller station with one time only that day, but don't want to get stranded

I wonder which site you have used to find this route from Siena to Rome and when you are travelling. At decent hours this smaller station can be either Chiusi-Chianciano or Grosseto and both are served by several daily trains to Rome.