Many alpines will happily seed into gravel, and lewisias of the cotyledon type come especially to mind so small plants inserted should make themselves at home quickly. Of course things like thymes and Mentha requienii also love to grow anywhere they've placed themselves. Asteraceae such as erigerons, asters, many others will be happy in gravel and establish by seeding there as well. Try anything that's easy and seeds readily.
How much rock? Any amount from total outcrop with minute planting crevices, to none at all - the billiard table look. While rocks look good if well placed, they can be very expensive and if the space is small, each rock means fewer plants. Good drainage is more important than rocks, great though they may be.
