I use 2cell LiPo packs exclusively, with voltage regulators to keep the voltage at 5v
pr 6v depending upon my servos. It is not just the voltage, but the capacity, as well. Be sure you have more than enough capacity to handle the worst case scenario where all the servos are stalled at the same time. This is where the highest current draw occurs. Get a Wattmeter or an H9 Current Meter.