Best think you can do is to go with the 6 volts system plus a 5.9 voltage regulator to keep it at 6 volts to protect your small servos and the receiver, the 5 cell packs are not 6 volt packs as the can go up to 7.5 volts when they are at its best and could damage your small servos and most important the receiver, the regulator will also make your battery last longer, when you run the servos at 6 volts your are getting the maximum performance out of the servos.

Hope this helps.

Have fun flying