If you can keep the external power source in the 8-10 volt range you should be able to handle that much current no problem. On USB power you are near the limit as there is a 500ma thermofuse on the USB source.
Of course an idependent +5vdc power supply can always be used to power the external loads, just be sure to wire the ground side of the external +5vdc to the Arduino ground pin.
Lefty