A second class ticket from Roma Termini to Venezia Santa Lucia is about E 45.00. What you'll have to take into account is the travel from FCO to Termini on the Leonardo Express, another E 9.50. If you miss your scheduled train, I believe you can reschedule for a later train, but seats may be filled for that day so you'd have to wait. Or you can just get on the next one, but you may not be guaranteed a seat. What if you stayed in Roma for a few days, then took the train out to Venezia so as to make sure you don't miss the train when you land?

The Eurostar train from Termini to Venezia S.L. will take about 4.5 hrs. Plus the other half hour to get to Termini from FCO and the haul until the train leaves to Venezia. Could add another 6 - 7 hrs total travel time after being on a flight.

I'd look at the low cost carriers and see if you can fly into Venezia at a reasonable fare, it may save you travel time. I am unsure of the train travel from/to the airport to the island, but Trenitalia might have that as well. If you get a rental car at the Venezia airport, then you'll have to leave your car at the big parking lot after crossing the bridge to the island anyways.

Again, the train will be a long-ish trip, but its a neat way to see the countryside. Is your itinerary set to go to Venezia first? Why not stay in Roma first, go up to Venezia, then make your way back down to Firenze then end up in Roma to fly out?