Futaba does not have a 6V rating. These are earlier digital servos that had trouble operating with a 5V NiCd pack. If you have a regulator that will regulate slightly below 6V, then the servos will be OK. 4-cell NiCd packs can peak at 6V for a bit. The main problem is with a 5-cell pack that can peak at 7.5 V. That's where the problem lies.

Most people call 5-cell packs "6V" even though they usually run a bit higher.