The musculature of the shoulders is very dependent on genetics and early developmental activities. Without the genetics for large shoulder muscle mass, it is very difficult to develop mass. And if the client was not big on athletics in their youth, it is even hard to gain size. And overuse injuries to the shoulders are very common. So, attempting to build the shoulders “quickly” is a very bad idea. Also, the old school lateral raise should be performed with the arms move about 5 degrees forward of the frontal plane. And as the rear delt is almost always the smallest and least developed of the visible shoulder muscles, it should be a focus muscle. But not to the point of overuse. Each client will present with different ROM, current developement, etc. Take time to assess shoulder movement and changes in shoulder movement as you proceed. Regression is extremely important with the shoulders. Failure to recognize signs of developing injury and ROM loss, can result in a long term issue.
The best advice would be go slow, progress slow, and avoid high repetition of any exercise. Be realistic and straight forward with clients wanting to “quickly” gain shoulder size/strength. Fast is not wise when it comes the the highly mobile low stability of the shoulders.