The quickest and easiest method would (IMO) be to book a seat on the direct train from Munich to Venice. It departs Munich at 11:35 and arrives at Venezia Santa Lucia at 18:10 (time 6H:35M). You might be able to get a Saver Fare. Check either the DB website or Trainline.com.
For the trip from Venezia S.L. to Firenze S.M. Novella, book a seat on either the Freccia or Italo high speed trains. The trip is 2H:10M. You can save money with advance purchase tickets on that train, but you MUST be willing to commit to a specific departure. The tickets are specific to train, date and departure time and can only be used on the one train listed on the ticket. You'll be assigned reserved coach and seat numbers.
The Eurail Pass will not be any benefit especially in Italy. The Passes do not include the compulsory seat reservations on the express trains, which you'll have to pay for out-of-pocket. Don't be caught on an express train without valid reservations for that train, or you'll be subject to hefty fines!