Even though Nimh/Deac batteries are nominaly 1.2volts a five cell pack will go up to over 8 volts fresh off the charger, they will drop back immediatly to a more stable 7 volts, however the A123's will come off the charger at 7.2volts and drop back to a stable 7volts volts without any discharge, most Rx's are rated at up to 20volts on the bus, and most servo's will take 7 volts without problems, though check the manufactures spec (Futaba 3152's are rated at 4.8v only, although I have run them on 5cells and A123's), once you start loading the A123's they will drop back to a very stable 6.6volts.

So in short the answer to your question is 6.6volts is not high enough to damage either the Rx or the Servo's.

Mike