6 motors geared for 20 fps will draw a LOT of current at stall (when starting, pushing, or turning in a skid steer arrangement). More than I'd want to design for; you risk resetting the cRIO (if you use the original 8 slot model) or the wireless gaming adapter due to momentary drops in supply voltage when drawing 600+ amps of current. You will then be sitting lame on the field for 20+ seconds while the component reboots.
With 4 motors, 20 fps will result in a woefully slowly accelerating bot.
Both of the above statements assume a bot at or near the weight limit plus bumpers and battery - obviously, a robot that weighs 80 lbs soaking wet can be geared faster and still accelerate at a good clip (although since it is soaking wet, maybe not). Maybe those crazy Cheesy Poof guys who design ridiculously fast welterweight bots are on to something
