Thanks again for all replies!
After doing further research, our current itinerary is:
Fly into Milan on a Tuesday morning.
Take the high speed train from Milan to Florence. Based on suggestions we have read here in the forum, we will not purchase advance tickets due to the uncertainty of when we will actually arrive in Milano Centrale.
We will spend the rest of Tuesday, as well as Wednesday through Saturday in Florence. (We switched Venice and Florence reasoning that more museums in Florence would be open on Wed. through Sat. vs. our earlier plan of being in Florence from Sat. to Tues.)
While in Florence we may take a day trip to Pisa.
Saturday morning we will take the high speed train to Venice. These tickets we will buy in advance since we can set our own schedule. We'll be in Venice the rest of that day, as well as Sunday and Monday.
Tuesday we will take the train from Venice to Milan.
We were considering stopping along the way (for a few hours) in Verona, or perhaps Padua or Vicenza. We gather that if we take the high speed train to Verona we need to purchase two tickets - one from Venice to Verona and a second from Verona to Milan. The regional train also travels there but it seems to take twice as long.
We will spend the next three days based out of Milan, but plan to take a trip to Lake Como and/or Lake Garda. We will have to figure out the easiest way to get there by train. We might also take the Viator day trip Bernina Express tour of the Swiss Alps.
Our next step is to purchase the train tickets from Florence to Venice, and then Venice to Milan (via Verona).
Before we purchase them, are there any other suggestions you might offer, or any "gotchas" we haven't anticipated?
Thank you!