Zermatt to Florence

You can search the Swiss rail or the DB sites to get an idea of how much time it will take and how many times you would have to change trains :

https://www.bahn.com/en/view/index.shtml
https://www.sbb.ch/en/home.html

It is a long ride minimum 6 hours.
Are you going to have some kind of pass e.g. Swiss Travel Pass or Half Fare Card?
The answer to that question will determine what parts of the route that you need to pay ahead of time.

Where did or will you purchase your ticket TO Zermatt? Buy a R/T ticket and that part is solved. The railway that takes you up to Zermatt is a separate railway from the rest of Switzerland. Try not to purchase your forward tickets to Florence in Switzerland as it can be very expensive. At some point, like when you purchase your tickets to Zermatt, you can purchase your ticket from Brig to Domodossola, the border crossing. Use an app like Trenitalia or Trainline to purchase the remaining portion of your train travel to Florence.

According to the Rail Europe site, you could reasonably find a journey with two to three changes changes taking 5 1/2 to 7 hours depending on the route. But it seems you’d need to change in Visp then Milan for the two changes or Visp, then Brig, then Milan for three changes. If you search on Rail Europe it will list all your options and which company is running each leg. You could use that information to book directly with the train company as I think rail Europe had an up charge and service fee.

Deutsche Bahn’s website shows a 5h 45m journey requiring two connections. One in Visp, CH and again at Milano Centrale.

Thank you all very much. I'll continue my research with direction now. Very helpful... we don't go until October. Pam

Don't buy seperate ticket if you can avoid it. With a single ticket for the entire trip it means all your connections are protected.