Organized campsites are few and far between, but you'll soon get used to just finding a flat spot of ground a little way off the trail and using it as a campsite. There are very few places where you won't easily be able to find a spot to lie down, and a lot of people end up cowboy camping (just laying down a ground sheet and sleeping bag without bothering with the tent or tarp) for a lot of the time.
Almost everyone wears trail runners rather than boots, for the reasons already mentioned. The only people who have problems with this are those whose packs are heavy. The PCT is a trail where most people go pretty light-weight, but if you expect to have a total pack weight over 35lbs for long periods, you should probably wear boots to save your ankles.
Definitely don't carry any kind of weapon - they're illegal in some areas through which the trail passes, and they're so heavy that you'll quickly regret carrying it. A lot of people are trimming the edges off their sleeping pads and mailing away spare underwear by the end of the first month - several pounds of firearm will just seem like an unsufferable burden, and - right or wrong - will probably make your fellow hikers feel uncomfortabale around you.