Answer: It sounds like you've done enough preliminary research to see that there are a lot of rock garden plants out there!
Do the people who "gave" you this project give you any guidelines to work with? If not, take a look at some basic rock gardening books to get some ideas for what might work -- leaving some open space, using abundant groundcover plants to fill in space, etc. Groundcovers take up space over time and eliminate the need to purchase a lot of plants! Also, do you want to make paths through the garden, with stepping stones or other material, to allow access? Draw some sketches of what you believe will work.
Here are just a few hardy plants to consider:
groundcover -- creeping thyme, creeping phlox, ajuga, and many sedums, laminum, euphorbia
Succulents -- non-spreading sedums, sempervivens
Flowering plants -- dianthus, achillea, armeria, Alyssum saxatile, hepatica, geum, California poppies, potentilla, tiarella
A good book on this subject is "Alpines for Your Garden," by Alan Bloom. Unfortunately, it's out of print, but you library may have a copy. Some good sources for plants are Bluestone Perennials, 7211 Middle Ridge Rd., Madison, OH 44057, ph# 800/852-5243 and Arrowhead Alpines, PO Box 857, Fowlerville, MI 48836, ph# 517-223-3581. There are also alpine/rock gardening organizations that may be able to offer you some advice: National Alpine & Rock Garden Society, http://www.nargs.org/
There's always the option of having a professional or a professional-in-training help you with the design, too. Best of luck! I'd love to see a picture when it's done!
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