I don't know if looking at playmobil's suggestions would help... they would have you put two different colors of leaf pieces on the same tree, which just looks silly to me. I think your individual trees look pretty good, actually. The taller light green one is the best.
As for creating a realistic forest, I struggle with that. But I remember one person here did an interesting, though complicated process one time with a great result. She lined up the trees she had, took a picture, and printed it out life-size. Then she set it up as a backdrop, arranged the trees differently in front of it, and took another picture, repeating the process until she had a multi-layered backdrop that looked really good behind the few actual trees she had. It was pretty cool.