I have Sig Kougar with Hitec 325s on the aileron, throttle, elevator and the 635 for the rudder and nose wheel. If your elevator control surface is big, I would use the 635 on it. Since one servo controls the rudder and the nose wheel I would use the 635 there as well, since it has to control two surfaces (more torque needed).

325s - ailerons, throttle
635s - rudder/nose wheel, elevator

If both ailerons are controlled by one servo I would use the 635 there instead of the elevator and put a 325 on the elevator.