Nor stupid, nor smart. You do what you want.
Trains run during holidays and there are no traffic jams on the tracks.
Also trains never sell out all the seats between Florence and Rome. There are at least 2 to 4 trains per hour traveling each direction and each train carries nearly 500 passengers. Tickets are seat specific, so if you have a ticket, you will have a place to sit too.
Occasionally, they run out of standard class (formerly 2nd class) seats for the very next train, in which case you just wait for the following train 10 to 30 min later or just buy a ticket in a higher class (like premium or business), which never sell out. In my experience, going to Florence every year, the only time I've seen the standard class sell out was in summer for the early morning trains. But even then it's rare. I generally just wait for the next train, because in the morning there is a train to Rome every 15 min. If you buy the ticket ahead of time, even just a day or two earlier, there will be no risks of running out of standard seats.
Easter Monday is a holiday when Italian families like to take a 'gita fuori porta' (out of city gates trip). Basically it's a quick trip nearby by car (for example Florence to San Gimignano). There is traffic on the roads for that reason, especially on Monday evening when people come back home to the city. But since you are not driving, that doesn't affect you.