Gen. Admission + Fast Pass @ Fuji-Q Highland

We will be visiting Fuji-Q Highland. Would buying General Admission and then the preferred (fast) pass for two roller coasters work?

I am not sure if I want to buy a 1-day pass since I feel like we won't ride our money's worth. We want to ride 1-2 coasters, enter the haunted house and the Attack on Titan ride. However. we know the haunted house and the AoT ride aren't covered by the 1-day pass.

My plan is to arrive almost 1 hour before opening time at the park (already reserved seats on an early express bus from Shinjuku station) and buy general admission tickets.

Then go to the preferred pass booth by the merry-go-round to buy preferred passes for two roller coasters. I heard it is $10 each but this is with a 1-day pass. How does it work if I have only general admission. Do I have to pay $10 for the ride and then $10 for the preferred pass? Can I even buy a preferred pass with only general admission?

Please let me know,

Thank you :)

PS - Also, does my plan sound possible? We are going 2nd week of April on a Wed.