Assuming your using a 2 cell lipo, the batteries voltage would be 7.4V nominal, not 6.0V. A fully charged 2 cell would be around 8.4V, you may risk damaging the stock servos and receiver by running that high of a voltage. You could run a lipo if you used a voltage regulator to step it down to 6V.
Fail-safe plugs in between your throttle servo and the receiver. It's designed to move the throttle servo back to the idle position so your truck does not run away on you if your receiver loses signal or voltage on the battery goes too low.