some good questions here.....as everyone has said you can mix analog and digital on the same plane.....but not on the same control surface like a digital on 1/2 of an elevator and analog on the other....you always want the same servo on the same control surface...ie 645's on both halfs of a control.....
I set up my bipe with 4 ailerons servos...with both lowers on one channel with a Y and the upper half on another channel with a Y....that way if I have a servo failure I have at least on good set of ailerons......