Vertical spinners very rarely stall because the mass of a robot alone is not sufficient to stall the motor - and the robot does not balance on the end of the weapon for very long.
Horizontal spinners may well stall, especially if they hit a robot which is against the wall and cannot be moved out of the way.
If the teeth of a weapon embed themselves in the target robot the motor will almost certainly stall!
If you need a very small motor for your design, you will have to accept (relatively) low power. That's just the laws of physics for you.
If you want a high-power weapon, you will need a bigger, heavier motor, bigger , heavier speed controller - and some decent batteries!
Don't forget that with outrunner type motors you can fix the weapon to the motor case instad of to the shaft, since the case rotates with the shaft. Then you can grind off the shaft and probably reduce the motor length by 10-15mm.
