For fast coverage, I'd plant about 24" apart but you can space as wide as 48" -- each plant will eventually spread to cover a 4-6 square foot area. The individual stems do not root - this is more of a very low spreading shrub rather than a root-as-you-go type of groundcover. You can encounter some rooting but only if the stem is allowed to lay on the soil for an extended period and even be covered by soil to some degree - this is known as layering and is a type of propagation often used with woody shrubs and vines.

Most cotoneasters will tolerate light shade fine but best growth and heaviest flowering/berry production will be achieved in full sun.

Might benefit from some shade in Southern California.

Good call!! In SoCal, shade is fine :-)