Boarding at next station - Can I board at Florence instead of Rome? - Trenitalia FRECCIAROSSA

Hello Eurail Experts,

I am planning to travel from Rome to Venice. I already reserved the seats for Rome to Venice. As a plan B, I would like to visit Pisa and board the same train from Florence. Is this allowed in Trenitalia FRECCIAROSSA. Will my seat reservations still valid if I board at Florence instead of Rome?

Thanks in advance!