Maximum current supply tolerable by 1A Motor driver IC
A 1 Amp supply would be appropriate for the chip. For the overall system, ... that depends on the "magnetic torque rod" and how you are driving it  (on/off or PWM) you haven't told us anything about that.

You can simply connect the "Return" pins to ground. 

Their purpose is to allow you to connect them to ground with a low value resistor (0.01 to 0.1 ohm) - then the voltage across that resistor tells you the actual current supplied to the load.  

If you don't need to measure the current, then just connect them to ground.