One-way-ticket is the right option to book passholder reservations in this case.

But don’t forget to add “Interrail / Eurail” as a discount before!

And what "seat reservation only" is for?  

Hello @Tommaso Ansaloni 

On some lines, you need to buy a surcharge when you have an Interrail Pass, for example for the Brenner. If you buy a ticket with the Interrail discount, you are sure to get a valid reservation.

“Seat reservation only” is for regular tickets,

It’s for people who have a ticket (including surcharge, if necessary, in this case) that is not fixed to a specific departure. 
 

If you bought the supplement but not a reservation, or if you bought the supplement & reservation but now want to swap to a different train, this is what you’d need - seat reservation only.