Mix the different rock, set them up and lean them against one another. That'll give you more height, crevices, nooks and crannies, perhaps a little greenery. I get my artificial greenery at places like Hobby Lobby or Michael's Hobbies. You can find unusual plastic greenery and a fraction of the price. Natural plants add a nice touch too, while they last. You have a nice rock collection maybe just re-distribute them an unorganized manner. Enjoy your work in progress!