Can I book 6 passenger couchettes from Zurizh to Salzburg using 2nd class ticket for 5 passengers?

I am planning to travel with my family (3 teens - 17/15/13 years old) from Zurich to Salzburg using night train. Can we book the 6 passenger couchettes for ourselves (1 bed empty) using 2nd class ticket? If yes how much the reservation cost do we have to pay? (6x11USD or 3x28USD) -
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