To answer all the questions takes a little understanding. Typically servos have three wires ground, power, and signal. The power and ground can be separate and most people as a matter of convenience draw power from the radio. A radio and servo have a certain internal resistance and as you increase the voltage you are only increasing the heat the unit will produce unit something gives. While you can increase the power to a servo, you will shorten the life of the servo’s motor. At some point the tiny wires inside the motor will overheat and stop working. Best is to switch to a heavier servo if you really need the torque.
Servo motors are D.C. type (Direct Current) and the have permanent magnets inside. The coil (armature) is also a magnet created by the voltage applied. If you have one strong magnet (the armature) and one weak magnet (the permanent magnets) then you really can not get more torque.
Batteries come in a variety of flavors distinguished by the number of “cells”. Typically, 1.2 volts per cell, and the current varies. What you are looking for is the highest number of “amp hours”. This says how much current you can have for what time period. R/C batteries are marked in MAH or mAh or Milli-Amp-Hours. So a 2400 mAh battery provides 2.4 amps for one hour.
D.C. or direct current unlike A.C., goes through the entire cross section of the wire. If the wire is too small over a long run, then the wire will act as a resistor and you will have a voltage drop at the end of the wire. The wire will also get warm.
If you plan to use a PIC, take a look at how much current it will handle. If it will not handle the necessary power, you may wish to use an “optical isolator” on the PIC’s output. This way the PIC only sees a low current L.E.D.
Usually for applications where you have a 12 volt supply and only need 6 volts, an I. C. device like a 7806 works well. Be sure to use the recommended capacitors so it won’t oscillate. You can get fixed voltage regulators that will handle up to 5 amps each. No, you can’t easily run them in parallel. Sorry.