I agree with the previous poster Carol now retired that there is not a need in this case to involve a third party website no matter how good Mark's website is.
It is really very very simple. Your train, no matter whether you use Italo trains or Trenitalia trains, will be fast trains with a couple of intermediate stops, but the key thing is you are joining in Venice at the origination station, Venezia Santa Lucia, and you will be going to Florence, Firenze SMN. While Florence is not the termination station for your train it is a dead end station which means that there is a fairly long stop between arrival and departure, giving you loads of time to leave the training. At both stations, because they are both dead end stations, all the platforms (tracks) are side-by-side so very easy to enter and leave.
Use whichever of the two railway companies you prefer, you may look at the photographs on seat 61 to help you choose but they will both be very similar for what you are doing. Pick the time you want to go at the fare you want to pay and just buy your ticket in advance - it will include a seat reservation and you will have lots of time in Venice to find your seat and once you get to Florence have your luggage gathered up and get off the train. The doors to either will have very limited step up and down, to and from the train, onto the platform as they both operate very modern trains which go very fast and have all the modern amenities.