|> I live in north Los Angeles county which I believe is Sunset magazine's zone
|> 18. Wonder how a native tree like Pinon pine would do?
Probably OK, though it won't like the shade much. But you will
have trouble with the problem that ANY conifer that is naturally
small enough will also grow slowly, and none like being pruned
down to size. Pines especially dislike it. You can clip several
to make a hedgy plant, but they will not regrow from old wood, so
you can't cut out bits that have got 'lumpy' and get any regrowth.
I don't think that you can even clip pines, but I might be wrong.
The yews are shade tolerant, though they tend to like more moist
conditions than you may have, and can be kept pruned to small
sizes and any shape you like. I don't know which of the several
plausible species would do best with you.
There are also some yew relatives (e.g. Torreya), which I know