The longitudinal stability criterion for dynamic stability is actually the same as static stability. And it is that the aerodynamic center of the aircraft must be behind the c.g. of the aircraft. Now, this means that there will be a negative (nose-down) pitching moment simply caused by the force moment of the lift and weight. Now this moment has to be counteracted by something, and that is usually the horizontal tail.

You are correct that a more efficient design is to place the c.g. at, if not behind the aerodynamic center as this reduces the so-called trim drag penalties. However, this can only be done if a stability augmentation system is used.

R/C aircraft can be designed with lower stability than manned aircraft. In fact there have been tests where large, manned aircraft have been controlled with the c.g. behind the aerodynamic center without augmentation. However, this only works if the aircraft has a large pitching moment of inertia (i.e. the time to double amplitude is high, so the pilot has time to respond). With small r/c aircraft I seriously doubt that anyone could control the aircraft with the c.g. behind the aerodynamic center.