Hello.
You can power the servos separately from the Maestro board (either directly or through some other power rail as you mentioned). If you do this, you would need to make sure that the servo power still shares a common ground with the Maestro board.
Alternatively, there is a trace on the bottom of the board that can be cut to separate the power rail into two, allowing for different voltages to be used on each. The cuttable trace is indicated by the dashed line in this picture:
-Brandon