Italo train tickets

Trains do not typically sell out. You should have no problem buying a ticket, It is not recommended to buy in advance as you cannot predict that your plane will arrive on time. If this is a direct flight from the US, you will have to go through passport control, and that is unpredictable as well.

Take the bus or taxi to Venezia Mestre station. Looks like there is an Italo train to Florence every hour. Cost is about 122 EUR for 2 people in Economy class.

Whether you want to spend the night in Venice, that is up to you. In that case, you would probably want to take the bus to Piazzale Roma in Venice, cross the bridge to the Venezia Santa Lucia station and get a hotel near there. You could then buy an advance ticket from Italo departing from that station and save some money there.

Trains are mass transit, if they'd "typically" sold out without railway companies adding more cars... they wouldn't be mass transit anymore .

That said, be aware that both April 25 and May 1 are national holydays: if you are travelling around those days getting a cheap ticket on a specific train could be more difficult than usual. There are always seats in Executive class, but those posh seats are three times more expensive than the others! For a family it's a big difference.

If I were you I wouldn't get hung up on Italotreno, take the first/cheapest high speed train going south. Trenitalia or Italotreno... there is no real difference.

You can go on the same day. The advice to wait until you arrive to purchase the tickets is a good one. You never know what snags there might be with your flight, getting through passport control, etc. Both Italo and Trenitalia have many daily high-speed departures from Venice to Florence, so you likely won't have a problem securing tickets in April.

As always thank you for your replies