Pull the red wires from each of the ESCs, and use a separate RX battery for power to the RX and servos (be it Nicad, NiMH or LifePo). Otherwise, you are asking for trouble relying on a BEC to handle the load of 7 servos.
And agreed with doing a Y on the batteries. You don't want one battery to go flat and give you adverse yaw. That will put you in a death spiral so fast you won't believe it.