With rail passes you still need to buy seat reservations for the faster trains. And it can happen that the day the train leaves there are no more seat reservations available for pass-holders if the train is popular. Since your itinerary is relatively fixed, you might consider buying those seat reservations ahead of time. For example, you could buy all your seat reservations for Italian trains when you arrive in Venice.

However, that would use up a significant hunk of your half-day in Venice. Either use your extra day in Venice or skip it.

Minar makes a good point about passholders needing seat reservations before boarding the train in some countries, like France, Italy and Spain - but in many countries like Germany, Belgium, Netherlands, Austria, the U K, Denmark, Switzerland there are nearly no trains requiring seat reservations and you can just hop on - in these countries a pass is sweet because of the total flexibility to just hop on any train anytime but in a few countries you must go to the ticket window and get a seat reservation - a hassle and one that limits sponteanity a bit IME.