Garden Landscape Design

I am trying to set out a small rock garden, as there is a large magnolia tree growing in the middle of the main garden and it has a lot of spidery roots. It is very hard to grow anything successfully because of these roots. What advice would you give me to grow cactus etc?

I have a small patch of garden in my retirement home with about 5cms of soil on top of clay. I would like to plant some suitable vegetables for myself to enjoy. What suggestions have you?