RE: WING LOADING APPROXIMATIONS
Trainers are usually < 20
Sport is usually 20-30
Scale depends on the airplane and the scale. But ultimately, the size of your field will determine how much real estate you have to land on (you can fly heavier planes on a bigger field). For scale, there is a very specific weight that will give you scale speed (call it the low end limit) and another specific weight that will give you a scale turning/looping circle (the high end limit). Anywhere in that range will give you an airplane that appears to fly "scale like". No matter the weight, it's the power loading that is most important to keep "scale".
The airfoil plays a large part in it too. A thick, high lift airfoil can produce nearly double what a thin, symmetrical wing can, equivalent to cutting your wing loading in half. likewise, if you have flaps, you can increase lift by 50%, or take a 45 wing loading down to about the same as a 30 wing loading with no flaps. I currently fly a B-24, which has a high aspect ratio, high lift wing with fowler flaps. It has a wing loading of 62, but it lands no faster than a sport model with a 30 wing loading.