can i go from florence to pisa and then pisa to nice?
"can i go from florence to pisa and then pisa to nice?"
Sure you can, in fact this is the shorter, most logical route.
If you're new to this read this how to find train schedules in Europe.
We left from La Spezia, changed to a French Thello train in Genoa, and went to Nice on that. Try putting in your departure and arrivals here. Keep in mind that there is a schedule update on June 9 and not everything after that date has been uploaded yet, so use a date before then to see examples. Use Italian station name for your departure point.
The option with least changes from Florence to Nice via Pisa departs from Firenze Campo di Marte at 07.36 and arrives in Nice-Ville at 15 57, with just the one change at Genova Piazza Principe. This is currently bookable six months in advance on http://www.trenitalia.com/tcom-en , at prices from only €24.90.
For other slower options via Pisa, enter Firenze S. M. Novella to Nice-Ville and a date before June 10 (later dates not yet available, but will be the same or similar. Options with a crossed shopping cart can be purchased from Trenitalia online only as far as Ventimiglia, A separate ticket is required for the French trains from Ventimiglia to Nice https://en.oui.sncf/en