7 servos, 2 for flaps. That means 2 for ailerons as well?

Personally, I think you'd be fine on a standard 600mAh pack, but you might only get 3 "normal" flights before needing a recharge.

Just make darn sure none of the servos, espeically the flaps, stall at the end of travel. Nothing discharges a battery faster than a servo stuck just short of where it wants to be for a while. That kind of thing is easy to do on throttles and flaps, where there is frequently a physical limit to the control movement that you want to match with the servo. On thottle, it's not that bad unless you roar around at full throttle all the time, but since most flying will be "flaps up", you have potential there for a problem. Note that this isn't specific to the size of your receiver pack, just a tangent worth going down .