The smart motor (V5 200 RPM) shows a slightly speed fluctuation when it launch, then it stabilize. We chek all the gears and there no component causing unnecessary friction. Is this normal because of the PID controller?
Yes. It sounds like the PID tuning on the motor is slightly underdamped for your configuration. Is this with the wheel off the ground or on the ground? You should see a different velocity response with the wheel on or off the ground, and the inertia of the robot should add some damping when the wheel is on the ground.
This is observed in the ground when you start to move forward or backward full throttle.