From personal and extensive experience I'd state that 6V flight packs (NICAD or NiMH ) are absolutely fine.

Why introduce a regulator when they are allready present in the RX and in some servos. Servos are faster and stroger on 6V, most manufacturers give specs for 4.8 and 6V.

If you wear / burn out a servo because running on a six volt supply then the servo was not man enouh for the job and would have been totally inadequate on 4.8V

Re battery capacity - see the servo manufacturers data on current consumption and rate batteries accordingly

current per servo x N servos = capacity in Mah

eg 500ma, x 4 = 2000mah, a 2000 mah battery will keep these servos happy for 1 hour, probably much more as you will not have all servos working at full power all together.

My Weston (Super) Couger has a 6V 600mah Nimh battery, after 4 10 - 12 min flights it usually takes c200 mah to recharge. I am using Futaba 9102 (ele), 9202 (rud) and 2 x JR 591 servos (ail)plus a hitec hs 81 on throttle.

Yes servos jitter a little on first power up after a charge but they soon settle down. Its not an issue.