Aziz,
I don't think it is a good idea to use 2 DS811 servos for Elevator halvs. It will be tail heavy. the plane has long tail moment.

I believe the Original at CA model website balances well with the stock locations for the servos.

Didn't you plan on using the bellcrank?
Unless you are nose heavy with 91FX, I suggest you not to use dual elevator servo setup.
If you really want to use dual servos for elevators, try to get mini digital servos with less weight.

91FX with fuel tank at CG will suffer from fuel delivery problem. this will cause lean run during vertical manuevers.

Right now, I can not access the CA model website, so I have no idea what else is possible.