RE: Solar Energy question?!
Daniel,
You may want to add a small backup battery for the receiver/servos, just in case you have a dropout of output from the solar cells so you can maintain radio control. Keep in mind just the shadow of a cloud or aircraft/bird could cause the output of the solar cells to drop out momentarily. Such a momentary dropout would cause you to lose control of the aircraft, not a good thing!
You also may be able to use a large capacitor to store output of the cells in the event of a dropout. If you use a battery for backup I would use a nicad rather than a li-pol due to the circuitry that you would have to use in the case of a li-pol to prevent charging the battery with the solar cells...
To isolate the engine power and esc from the receiver power circuit all you need do is cut the +5v lead from the esc to the receiver, you may want to use a short extension and cut that lead on the extension and leave the esc wires intact.
What type if wattage are you planning in using for the aircraft power?