I wouldn't use a Lipo on a combat model, IMO too much of a fire risk. As far as voltage goes, some older servos have issues with anything over 6 volts. I almost lost a helicopter a couple months ago running 6.6V LiFe on older Futaba digital servos. I also had to throw a regulator into a sailplane to use 6.6V on some older JR wing servos. Your best bet is 5 cell MiHM.