[Woolly]

If you're going 4 wheel skid, the sides of the robot you're driving from need to be shorter than the non-driven sides, otherwise your maneuverability will be compromised.

Also, I'd say that you should be fully prepared to use chain or belts. Each side of the skid steer should probably have 2-3 CIMs though the same shifting gearbox that directly drives 1 wheel, and then chains or belts to the others on that side.
I'd try start with Vexpro gearboxes to begin with, and in future years upgrade to West Coast Products gearboxes if you need that little extra.

We're also tossing around the idea to do some drive train R&D this summer, and we're looking to make a 2-speed 8 Wheel Drive with placation wheels and a belt drive. Potentially we would put omnis on the outer 4 wheels if we want to trade some resistance to defense for maneuverability, but who knows what we'll find.