1) The bobbin thread should be the same size as the top thread because larger top thread could cut smaller bobbin thread, and vice versa.
2) No, it doesn't matter what color the bobbin thread is.
3) Your Juki should be able to sew with size 270 thread with a size 24 or 25 needle.
4) Yes, it's done all the time.
5) You can clip it pin it, whatever, but I have had the best results with glue. Glue one side only with foam to foam glue, just enough to hold the fabric to the foam while you sew around the perimeter of your pattern line.
6) Steam fabric, but never leather or vinyl, and then only as a last resort.
7)The vinyl will soften up when you put the seat cover on, and most vinyl should come in a roll and not be wrinkled up anyway.
8) About 2 1/2 to 3 yards depending on the seat.
9) Juki is a great machine, a step up from the Singers and Consews of the world, but not as good as Adler or Pfaff.
Check out this thread, it will be exactly what you need for your class: Truck Seat Upholstery Tutorial