Nocoalee35 I would prefer to keep just the St. Moritz-Tirano protion first class.
No problem, in Switzerland you can buy a route-specific class upgrade for a specific leg of your journey with a 2nd class Eurail Pass, e.g. only St. Moritz - Tirano. I don’t think that it is possible in every country.
https://www.sbb.ch/en/offers/class-upgrade
No seat reservations are necessary in Switzerland, you can find more information here:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
An exception is the Bernina line which suffers from overtourism, it might be advisable to reserve seats even on a regional train from Tirano to St. Moritz. Access to platforms might be restricted if you don’t have a reservation and you have to wait for the next available train.
I recommend to reserve seats between Switzerland and Germany and for long-distance trains in Germany. If you reserve on bahn.de (German Railways) you can usually choose seats on a map. Be aware that some seats designated with an exclamation mark are between two windows. More information here:
https://www.seat61.com/interrail-and-eurail-reservations.htm
You can also reserve seats for two consecutive days for the same price, e.g. Passau - Munich - Basel by adding the intermediate city as stopover. Adapt the length of the stay until you find the right trains:
