[Ty Tremblay]

Though a 4 speed robot would be cool to see, my first suggestion would be to eliminate your transmission and get your speed and torque changes by a difference in the diameter of your wheels. If you have a small traction wheel and a large omni wheel, you'll get pushing power when in traction mode and speed when in omni mode. This will reduce complexity and give you more room to mount your pneumatic pistons for actuating the modules by making the modules smaller.