The journey from Zürich to Wien with the train leaving at 6.35 will involve 4 different train so you need to look for the individual trains an input them as journeys.

The first train is Zürich 6.35 to Sargans 7.33

Then Sargans 7.36 to Kufstein 10.51

Then Kufstein 11.00 to Salzburg 12.24 with a rail replacement bus

Abd finally Salzburg 12.52 to Wien 15.22

The train 6.40 shown in the Railplanner doesn't seem to run from Zürich tomorrow. Otherwise that is a direct train from Zürich to Kufstein. 

Never use the Eurail website or the rail planner app to plan. Only trust the timetable of the operators, in this case SBB and ÖBB, who show the same timetable.

So you can indeed take a train at 6:35, change at Sargans to another train, change at Kufstein to a replacement bus (pass is valid). The 3-minute change in Sargans will work because it's Switzerland.

The train Zurich-Sargans is in the rail planner app

You'll need to add Sargans-Kufstein (train) and Kufstein-Salzburg (bus) manually: scroll down on the planner results screen and tap on "Add it manually”.

Thanks.H’ever,for 7:36,no schedule are available on EuRail app?

You need to enter that manually, as @rvdborgt said in his reply.