Use a LiFe only. A LiPo will be too "hot" (high voltage) for the receivers/servos. But a LiFe, while pushing the voltage a bit, is fine with the radio gear. Also get a Tech Aero IBEC (
Ultra IBEC - Tech-Aero Designs LLC). Besides being a great ignition switch, and fantastic RF isolator, it has an in-line voltage regulator, so you can make sure the voltage going to the ignition never exceeds the 6v.
My 40cc with 5 high torque servos gets about 8 12-14 min. flights from a 2400 mah pack. Of course, your mileage may vary a lot from mine....