Take the train direct from Rome to Venice. The train takes about 4.5 hours so skip the stop in Florence - you can't do the city justice with a quick stopover. Check train schedules on Trenitalia - most trains continue on from Mestre to Venice's S. Lucia station.

The one hour flight time flying from Rome to Treviso is a bit deceiving. The time and expense is much more once you factor in the time & money getting to/from the airport and the time you must be at the airport before your flight departs. Most of the airlines flying this route have very strict weight limits on luggage. If you go over the amount you will pay dearly. The train stations are located in the center of each city.