Japanese would call this Saikei - a miniature landscape where rocks are used to give substance where the trees are immature. Can be a great way to make use of young trees until they develop enough to become bonsai by themselves.
You've also managed to create a classic beginner group with the trunks almost evenly spaced. Group plantings look much more natural where the spaces between the trunks varies to look more like a natural forest.
I do like the much more natural looking rock outcrop.
Just be aware that rocks not only take up space in the pot but also absorb water from the soil and radiate it into the air. Pots with rocks need more water, more often than similar pots without rocks.