To me the rocks are all sort of the same "too big" size. That "tree" doesn't work at all in there, especially considering the size of the rock. With the five stones, I would do two groups, one with two stones and one with three. Arrange to minimize size and to portray a "eroded" feeling. Those stones are very sharp and young looking, it will be difficult to find an angle that makes them appear weathered . Good luck, I think you have good intuition for this.