Hello.
If you are driving the wheels directly in a balancing robot like that, I suspect you will want to use microstepping for smoother movement, which generally requires using a driver with current limiting. To use the #1208 motor with a 2s LiPo and do microstepping you would have to set a low current limit . The maximum supply voltage that can be used with a given motor depends on the characteristics of the driver used so I cannot provide much guidance for your system there. It is common practice to use higher voltage power supplies with current limiting drivers in systems with motors that have lower voltage coils. For example, the pull out torque curve in the datasheet for our lowest voltage NEMA 14 motor (our#1209 motor) is for a 24V supply (torque decreases quicker with speed when using lower voltage supplies).
We only have a few drivers that have a minimum supply voltage that would work with a 2s LiPo and only our DRV8834 driver carrier would work with a 1s LiPo, so you should pay attention to the minimum voltage specification when selecting a driver. Also, you should avoid using the DRV8825 with a motor with a low coil voltage (like the #1209 motor I mentioned earlier) as it can have trouble maintaining the low current required for accurate microstepping with such motors.
-Nathan