We're due in at 7:30 am and I was looking at either booking the 9:35 or the 10:35 from Roma Termini to Venice.
For the 9.35, you would need to make the train from the airport at 8.38, and I wouldn't want to count on that for a non-changeable ticket. In this situation I'd buy the tickets on arrival, with some thought of getting a cheaper ticket allowing a long time to catch it, if not booking a flight.