Thank you very much, Roland & Danhiel, for your kind replies. I should have worded my question more carefully.
I will probably join the ECE 198 Munich-Zurich train (operated by DB) partway at Buchloe (Germany), & can see a reasonably priced DB ticket that I can buy for the German + Austrian sections up to St Margrethen SG (Switzerland).
I will probably buy the SBB Saver Day Pass (which looks like the most economical option) for travels from St Margrethen SG onwards – to Zurich & then southern Switzerland later that day.
I was just unsure if I could stay on the same ECE 198 train with a DB ticket up to St Margrethen SG, & a SBB ticket from St Margrethen onwards; I think I can probably stay on the same ECE 198 train if I have a valid ticket for each section of the journey, & just hope I am right in thinking so!