Hiking in a down jacket will likely be far too warm unless you hit some serious cold - so bring gear that you like and feel comfortable hiking with. There really isn't a right answer - some gear is lighter than others but ultimately you need to be the one who wears (and carries) the stuff.
In regards to a rain jacket - tt does rain in Southern California and people such as myself who've only hiked the trail once probably speak too much in absolutes. But I can say with certainty that it can and will rain in Southern California in late spring. Several years back there famously were some hikers who were on Mount Baden-Powell and got caught int he snow and rain. They were hypothermic. They had sent their tents and rain gear ahead and then called a trail angel to be picked up at nearby road. The trail angel refused, citing their lack of preparedness wasn't a reason to need "rescue." This resulted in quite a bit of gnashing of teeth over at the PCT List Serve (pct-l). But ultimately, the fact is that you are responsible for yourself and even a lightweight rain jacket such as DriDucks or something on that line provides adequate protection. Personally, I'd like a heavier duty rain jacket if I was going to use if for days on end (see the state of Wasington), but many have carried DriDucks and jackets of similar ilk and been fine.
Yes, you are likely to carry rain gear for many miles without using. You can always send it ahead to a post office if the forecast looks clear. Or you can hike right through bad weather or even wait the bad weather out. It generally doesn't last long. But my jacket made a nice wind breaker a few times in Southern California near the wind farms, where, it can really, really be windy (go figure). It made for some nice fashion while doing laundry in town. Mostly, though, it wasn't needed until I reached Oregon.
Clint's suggestion for sleep socks is an excellent suggestion, particularly if you get cold. It also keeps your bag cleaner, particularly in SoCal where the dust and grime can build up in your shoes and there isn't always a lot of extra water around to wash up with.
More importantly, Clint point out that as a thru-hiker, gear weight is just but one consideration. He's right, you do adapt and if carrying a heavier jacket or thicker pad is the difference between enjoying your trip or not, by all means, carry the extra ounces. The point is to have fun out there and enjoy this opportunity to hike to your heart's content.
I admire your lightweight approach to things. I think the suggestions by Dan, Clint, Stephan and Scott are really insightful. Wish I knew more of this stuff when I started down the trail!