Hello Lucy
Welcome to MySwissAlps
You give us only very sparse details of your Swiss travel plans. What I am about to say can only be regarded as suggestions.
Base yourself in Zurich.
Two reasons. The train ride from Florence to Zurich is easy and quite quick. For example, if you were travelling today you could leave Florence at 08 55 and reach Zurich at 14 27. There would be one very easy change of train, at Milan Centrale, arrive 10 50, depart 11 10.
Secondly and more importantly, Zurich (https://www.myswissalps.com/town/zurich/) is at the centre of Switzerland’s outstanding public transport system. You can get to anywhere in Switzerland and back in one day.
Invest in a four-day Swiss Travel Pass (https://www.myswissalps.com/travel-ticket/swiss-travel-pass/)
On Day 1 get an Italian ticket for the journey Florence to CHIASSO. Chiasso is the frontier station 51 kms, (31 miles) north of Milan. Your STP will take you the rest of the way.
On Day 2 get an Intercity train to VISP via Bern, and then a local train to Zermatt. Walk across the road to the station for Gornergrat (https://www.myswissalps.com/activity/gornergrat/). Your STP will give you a 50% discount on this spectacular mountain line. Sit on the right for splendid views of the Matterhorn.
On Day 3 get an InterRegio train to Luzern, and then the Luzern-Interlaken Express to Interlaken Ost. Interlaken (https://www.myswissalps.com/town/interlaken/) is the centre of the Berner Oberland region. Your STP will take you FREE to Murren, Wengen and Grindelwald. To go up higher above these villages you will have to pay a (discounted) fare.
Returning from Interlaken to Zurich, take the quicker route using intercity trains via Bern.
Day 4 Go to St Moritz (https://www.myswissalps.com/town/st-moritz/) Go outwards via Chur and Filisur. Sit on the left when going from Zurich to Chur. Sit on the right when travelling on the Chur to St Moritz train. Return to Zurich via Klosters and Landquart.
Are you doing a lot of travelling in Italy or other European Countries?
If yes and you live in Europe, you should seriously consider getting a global Interrail Pass rather than an STP. If yes and you live outside Europe, you should seriously consider getting a global Eurail Pass (https://www.myswissalps.com/travel-ticket/eurail-global-pass/) rather than an STP.
Best wishes for your detailed planning.
Roger