I'm going to be driving 6 servos - (Hitec HS 645mg High Torque servos to be exact)
Half the time only 2 servos will be active, but at other times 4 or even occasionally all 6 will be running for a short period)
If you want the servos to hold their positions when inactive, you will still need to pulse them from the Arduino. Fortunately the Servo library makes this very simple and will continue to pulse your servos in the background with whatever position you last set.
In your application you might need a larger current power supply, I could not find the current specs for you chosen servos, but if all 6 of the are operating against a load they could draw 6+ amps.
For an example of using the servo library to control upto 12 Servos see my blog - scroll down to the second and third post. From the moment you call the attach function, the servo library will begin pulsing the servo to set and maintain a centered position, any position you set after this will be maintained in a similar fashion without your code having to do any work. This will ensure that your servos maintain their positions even under load in your project. rcarduino.blogspot.com