[VKP]

I think it would be beneficial for you to use four CIM motors, and instead of using casters, I would use omni-wheels, which can be driven, unlike casters.

If you had casters and only two CIMs, you would be pushed around the field very easily. Omniwheels and two more CIMs on your drivetrain would help a little, while maintaining the turning capabilities you want.

A question though: why do you want to power the wheels on the "arms" of the U and leave the back of the robot to drag around? I'm assuming that with a U-shaped chassis, much of your weight will be at the back, so you will want powered wheels closer to your center of gravity so you can turn faster.