I beleive that ESC is switchable 6v or 7.4V. To be safe, you should run the BEC to match the servo requirement (6v). It will more than likely work at the 7.4v, but you run the risk of overvoltage on the servo motor.

Also, the ESC will provide the voltage you have it set for to all other electronics and will source the current required (up to 5amps for the RSX).