you don't need a seperate lipo pack. well, you can do if you want, they have 7,4 volts, which means good cooling with the 6v fans, no problem at all for sure.
or, you plug the cable for the fan into the receiver. there you get, depending on the ubec/bec, about 5 or 6 volts. if the bec is powerful enough for the big servo, don't worry about the fans. just don't plug the 6v fans onto 10s...