Hi loopylulu,

Clay can be good and bad. There are a limiting amount of plants that can grow in just clay. Your idea of mixing sand and organic material will revamp the soil and help plants grow.

Clay has small pores and can become packed down and hard---with little or no drainage. This is devastating to plants they have little oxygen and drainage for their roots and usually dry out from compaction or rot from poor drainage. Try to mix in a generous amount and break up the clay. It is easier to mix after a dry period because clay is heavy and hard to mix when wet. Also, you may want to add some small gravel or crushed granite to areas with less drainage.

Or, If you go to the hardware or garden store you can find gypsum, this mineral helps break up clay as it penetrates through and breaks up soil. I have a ton of clay in my garden and recently used a product called "soil buster" which contained gypsum and fertilizer. It worked great, I even have vegetables growing in clay soil after adding organics, gypsum, and some mulch--pine needles, whatever is easiest to find.
If you are going to use a lot of big rocks, consider plant positioning before adding all of the rocks, where will they get the best sunlight etc..a rock can "live" wherever. Take a sunny day and watch how the sun changes on the area--make note of the amount of sun and how long each area is sunny.

You also might want to do a soil ph test. You can find inexpensive kits at the hardware or garden store, it's good to know your ph before planting because all plants have different ph requirements. Clay is most likely to be more alkaline than acidic as there is little organic material in clay. It would be cheaper than killing plants by accident and then having to buy new ones.

My best advice is to take your time and do your research. Since you have a small area, you can make a nice sketch and have fun designing your rock garden.
Hope this helps--Have Fun!!!
:) Firegurl
Danielle