[EricH]

Orientation: Long robot or wide robot? (Wide robot has the wheels on the short sides; long has them on the long sides.)

Center drop is used with >4WD. When you don't specify a drive, someone's probably going to assume a 6WD drop-center drivetrain and ask about the drop. (Typically, the center wheel is a fraction of an inch below the others in that configuration.)

I'm inclined to agree with BJC--you'll spin around the pneumatic wheels. (Well, a point on the axle of the pneumatics, at any rate.) AFAIK, there is no way to avoid that with a 2-traction, 2-omni setup of your type. You can avoid it by changing the placement of tractions and omnis, but they'd need to be the same size to pull it off effectively.