From my origin Milan is often much cheaper to fly to than Venice or Pisa, but it may be different for you. The usual guidance is to avoid flying home from Venice, because if you must make a connection in Europe before heading back across the Atlantic, you will have a very, very early departure from Venice. That means a painfully early wake-up call and probably costly transportation to the airport.
The places you list will certainly fill your time, perhaps over-fill it. You'll want to get up into the mountains above Bolzano, which is a very attractive city but down in the valley. It takes a good bit of time to deal with buses followed by lifts. If you want to be reasonably sure of having time for hiking, I'd suggest at least 3 nights in the Dolomites, which buys you a bit of insurance against bad weather.
The Cinque Terre are also somewhat time-consuming to get to. Whether both the Dolomites and the Cinque Terre can be squeezed into your trip (which is really 9 days not counting the initial jetlagged arrival day) will depend on how much you want to see in Tuscany and the number of days you want in Venice. You didn't mention a particular interest in art, so you may find that one day for Florence proper is OK if it means you can get to all the other places on your list.
If it were my trip, I'd definitely drop at least one of your proposed destinations to avoid spending so much time on trains/buses and checking in and out of hotels. You'll see more if you aren't running around so much.