Answering the pass question: you will need a First Class pass to cover your Glacier Express Day. So at a minimum, if you do not want to travel in First on any other day, you could but a Saver Day Pass in First to cover that ride. You can buy that now for the best price. A First Class Saver Day Pass bought now for March will be 88 CHF with no Half Fare Card or 59 CHF with a Half Fare Card. Full price for the Glacier Express from Zermatt to St. Moritz is 272 CHF in First, so this is a significant reduction.
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
https://glacierexpress.ch/en/price
For the rest of your travel, consider a Half Fare Card. You have at a minimum, travel from the Zurich Airport to Zermatt (regular price 134 CHF in 2d class, much more in First) and St. Moritz back to Zurich Airport (84 CHF in 2d class). The Half Fare Card costs 120 CHF and is good for price reductions in both First and 2d class. It will also give you 50% off other train and lift rides, such as the Gornergrat Bahn in Zermatt.
You do not need to have the HFC card in hand when you buy your Saver Day Pass or other tickets, but you must have it to show when you ride the train.
I would not bother with seat reservations on the other trains in March. Nor would we choose First for the other trains; they should not be nearly as crowded in March as they are in the summer high season when seats in 2d class can be scarce on popular routes.