Saver Day Pass is fine. It is an “all you can eat” public transit pass good for all public transit in Switzerland. When buying far in advance this is the cheapest option. With that you can travel to Zermatt, so you are all set.
The only thing you need to do to make sure you are on your desired train is to be at that train before it leaves. Train travel is mass transit. Think of it like a nationwide subway.
Note that if you bought a ticket with the Half Fare discount you also need to have the Half Fare Card.