Yeah, that would be ok too. 6.6v is a tad on the high side, but do-able considering that 6 NiMH cells is usually around 6.4ish volts.
I've seen my 5cells Rx packs come out of the charger sometimes at over 7V. I think most servos can handle 7.2 volts and I've seen some digital one even list it.
2s a123 would be 7.2V off the charger. A littel on the high side, but I have considered doing the same thing in the past for my nitros. They only reason Ididnt was the dang battery boxes on the nitros were just a touch to small to fit the length of the a123s.
I wound up using 2 of the 1200mah LiFePo4 cells in seriies though with no isses. Same voltages as the a123, just smaller size and capacity.
I can't decide if its more fun
to make it...
or break it...