Not sure about the 757 but IIRC the start sequence on the 767 was #2 first from a ground air source. The air connection is on the lefthand side and besides the obvious safety benefits it also allows you to use the isolation valve to cut off air to the starter should the start switch fail to release from the GND position. (Just in case your guy on the ground unit is to slow to react) If you started #1 first the only valve in the line is the start valve itself, your only protection should it fail is a quick response from the air cart operator.
The 737 should work the same with the positions reversed. i.e Ground connect on RHS, isolation valve to kill air on #1 engine if required.