Unless California begins to get some serious snowfall soon, it could prove to be a fairly dry year on the trail. Keep this in mind in Southern California, where there could be some rather long distances between water sources. Thus, you may need to carry a good deal of water. I think in 2009, a slightly lower-than-average snow year, I had to carry five+ liters at times, and almost always had four litters when it was hot in SoCal. So keep that in mind. You will need the capacity to carry water and just as important, to get water out of what may be a trickle rather than a stream.
On shorts - many people wear them, but if you are not used to the heat/sun do make sure you have a lot of sunscreen. And in Southern California you do run into a good deal of thorny plants, so keep that in mind. I had convertable pants, which I liked for the simple reason I didn't have to be as careful where there was overgrowth. Plus, I didn't have to wear as much suncreen and in the heat, I figured I would dehydrate less quickly. Really, people wear shorts, love shorts, it's just as a northerner, I wasn't used to the sun that time of year.
Good hat that blocks the sun and sunglasses are advisable.