[simplechamp]

Yes, go for the external BEC. You already have a 4-6S lipo voltage source powering the Flux, might as well just step it down and have a constant 7.4V for the RX and servo. Using a 2S RX pack will take more room, add another thing that you need to charge, will not be a constant voltage, and will require an external LVC device.

I don't know what brand of radio you have, Spektrum RX's can handle up to ~9V, not sure about other brands.