RE: Servo extension & retract servo
Retract servos are a different design than standard servos. They really have only 2 positions possible positions, retracted and extended. Normal servos moove over a basically infinite range (not really but close)
Also, the retract servos deadband is greater, so it is less likely to drain the flight pack at the ends of travel. In other words, within reason when it gets to the end of its travel in either the retracted or extended position, the servo motor stops. It would be very hard to get a normal servo to do this, but it is not impossible. Your linkages would have to be really perfect, not easy to do.
Having said this, many will tell you you can use regular servos......you could probabaly drive a car with your feet too, not sure either is a good idea.
Good luck