If you go to trenitalia's web site you can research schedules for the Freccia (fast trains) lines. I don't think Pisa is on that line but rather you would go through Florence on Freccia and switch to the regional train. Somebody correct me if I'm wrong, it's been a couple of years since I made that part of the trip. The regional train schedule won't be up until the middle of June. Check for the same day of the week for next week and you'll see an approximate schedule for the regional lines.
I recommend getting your Freccia ticket when you can commit to a time schedule. You will then be able to buy the most economical ticket, and probably get the better seat reservation. Florence to Venice gets real busy with early reservations, as does Rome to Florence. There are plenty of trains though.
Have a great trip! Ever consider spending less time in Florence and more in Venice? And Pisa could be a nice day trip from Florence. Of course it depends on your interests and past visits.