Doc

You can use any combination of pack as long as they are the same capacity, in fact I use 4S + 6S in one of my airframes to make up the ten as I can then use them in other airframes as well, as to the capacity that depends on the duration required and the Amps being pulled, if we assume 50amps (each motor) cruise then for a 10min flight we would need about 6000Mah if does not quite work out maths wise as more power will be needed to climb out and some capacity can not be used, go for the highest "C" rating you can to keep the work load on the batteries to a minimum, also higher "C" ratings will actually produce more power due to the lower resistance in the cells.

As an example of power to cell capacity, I have a EDF which uses a 4S 4350Mah and the duration is only about 4mins of flat out mayhem, but the same battery combined with a 6S 4350Mah in my 2.1M Composite Edge will give me over ten minutes of aerobatics, these are 25C FlightPower LiPo's.

Mike