Hi!
Your model is a 3-D model and as such isn't supposed to fly fast...But...If you want to reduce the change of getting flutter you do the following.
1. Get all linkages as tight as possible!
2. Use as long control arms (those at the elevator, rudder etc) as possible.
3. Use a short servo arms as possible.
4. Never let the ailerons get so long so they reach out to the wing tips. Stop 10-15cm from the wing tips!!!
5. Build the ailerons stiff!
6. Keep the gap between the flying surfaces tight (less than 0,5mm)!
6. Seal the gap between the flying surfaces with a strip of Oracover plastic film (best plastic film covering).


No mass balancing is necessary if the above advice is followed.