[EricH]

If I might make a suggestion:

You're a little worried about fishtailing, so why not NOT put omnis on only one end, or traction wheels only on one end? Why not put one omni on one end, and one omni on the opposite corner, and do the same with the traction wheels?

See: FRC494 in 2006.

This gives less fishtailing than two omnis on one end, similar stability to a 4WD with all traction wheels, and easier turning than said 4WD all-traction.