While this will certainly work in moving your robot around, there are a couple of potential problems with that. One, when turning, your robot may “stutter” unless you have omni wheels in the front. Two, if your back wheels lose traction, you will be immobilized. Three, it may be easier to tip over if your robot is back-heavy. If these problems do not pertain to you, I don’t see any reason that your configuration would cause any problems.
No. It is extraordinarily unlikely that your drive would break VEX gears. The only time VEX gears “break” (which usually means snapping a tooth off a 12-tooth gear) is when geared down a lot to lift a heavy load. I can’t imagine how you could break a gear in a drive train.
Most teams would probably have them set to torque so that they can be used with 269 motors as they have the same rotational speed (100 rpm), so that they can have multiple motors on each side of their drive train. If you try to use a 269 motor with a 393 in speed then something will probably break.
I used to have the 393’s on our robot set to speed, but i found that it made it a lot harder when you needed to change a motor in a hurry during a competition. Because the motors are set to strength by default, if they are supposed to be on strength on the robot you can just screw them in and you’re good to go. But if you need them on speed, then you’ll need to take the motor apart, change the gears and put it back together before you put it on the robot. I would recommend internally gearing for torque, and then if you want the motor to go faster, externally chain or gear for a faster ratio.
I updated this post to include a picture of one way you can match 393 160 rpm motors conviently with 100 rpm motors. Both 1508 and 599 teams use this configuration. It keeps the motors at the rear, keeps them low and allows simple gearing. The main draw back is the cantilever which can increase normal forces on the axles by a factor of 2 or so but it is a tradeoff we were willing to make.