RE: Flap / Elevator Mixing
It is not uncommon for the plane to balloon when you deploy the flaps. You can limit this by slowing down as much as possible before deploying the flaps
The question is: Once the flaps are deployed and the plane slows down and settles in, does it fly nicely? If it does, do not mix elevator into the flaps.
If however, you find that the plane wants to fly "Nose-Up" with the flaps deployed and you must constantly hold a little down to keep her level, then you can try adding about 6 - 8% down elevator to the flaps