The plants are beautiful and they are going to fill in beautifully. I like the idea of mixing up the size and type of rocks, I agree that when it is all the same it can look kind of sterile. The plants are the thing though and once they start taking up the space they want, moving in different directions, it will take on a life of its own. I also think if all the rock were the same I would be more obsessive about picking up every little thing that dropped in there and, you know, a little of nature's disorganization is a beautiful thing at times.
