Most servos were designed to effectively use the 4 cell batteries (4.8V nominal) but some wanted faster response and more power so started adding 1 cell after they found the servos would usually withstand the higher voltage. They do this at a slight risk of earlier servo failure and faster wearout time. The capacitors used in the servo are usually the weakest length as their breakdown voltage is quite low as, to get the necessary capacitance in a small package, insulation between plates is the bare minimum to minimize bulk. Until you get very proficient at flying, I'd recommend staying with the 4.8 volt batteries. As to the dual rate, that is usually a function built into the transmitter so that with the rate switch in one position, you get maximum throw on the servo. By throwing the dual rate switch the other way (and adjusting the amount of rate change) the throw of the servo is reduced. This is supposed to help the beginner by not allowing him/her to overcontrol the model.