Some great advise already given, so I'll just share my experiences with rock gardens.
I've done rock gardens 2 ways. 1) Terraced with a rock border and 2) Random rocks with planting pockets in between.
When I've had a freestanding mound I've prefered to do terraces to avoid the studded raisin cookie look
![:lol:](./images/smilies/icon_lol.gif)
But I've tried to make the terraces as natural looking as possible.
The random with pocket approach has been used several times on hillsides that needed some erosion protection where I also wanted flowers. The last one I did I just picked up the rocks and chucked them at the hill. Wherever they "stuck" is where they ended up. Then I put some good compost in between and planted. It ended up being fairly natural looking, but after a few years you could barely see the rocks since everything grew ove them so quickly
Another idea is that you might consider a shape other than just a circle. Making it a little freeform might help to give it a more natural look.
Good luck with it
Chris