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You wont loose much runtime using the main batts for servo/Rx power, but you will need a seperate BEC to power the servos via the lipos, since 7.4v is too much, ~6v maximum really. Try BrianG's wesbite for wiring diagrams:
http://www.scriptasylum.com/rc_speed/_wiring.html
The capacitor should be wired to the esc's power wires, close to the esc as possible, + to +, - to - (in parallel).
You will also have to disable the built-in BEC by removing the redwire from the rx lead plug, and taping it back onto the lead.