Would a pre-purchae of our tickets be recommended/necessary, or should we just wait until we get there? We are going the first part of July Thx in advance!
You absolutely can wait until you get there. There are 18 trains a day running from Florence to Venice. Each holds about 400 people. The easiest way is to purchase from the self-service kiosks in the station. They will prompt you in English, make your seat assignments and will accept your American credit card. Very easy.
Your alternative is to buy in advance from RailEurope which charges more for the ticket as they are ticket re-sellers.
You can also always try to buy it from the Trenitalia site, but lots and lots of people have trouble using US Credit cards on there. The advantage is that sometimes you can get a discount. I was able to do it about 2 years ago and got a 20% off Amica fare, but they've changed those discounts now.