So you are a little north of San Francisco and just a bit inland. Thanks for including your location, that helps. You should have really nice weather in the winter especially. Your chickens should not be confined to the coop much if at all.
I’ve seen numbers anywhere from 1 square foot to 16 square feet per chicken on this forum. Which one has the magic? None. There are just so many variables that you cannot go by any magic number. We are all unique. You might follow the link in my signature to get a feel for what I think some of those variables are.
I like what the others said. You mention that they free range but you don’t mention if you keep them locked up at night for predator protection or what time you might open the door to let them out. Unless you are down there every day of the year to let them out at the crack of dawn, you may fine you have a problem if you crowd them much more. That’s one of those management practices that can make a big difference.
I realize you said you would keep them in a separate coop until they start to lay, which would help reduce the problems with them trying to keep a separate mini-flock, but I’d still be extremely nervous about adding more than two more unless you leave it open so they can get in and out at all times, which means predators can get in and out at any time. And you’d still have to go through integration. The way you are talking about keeping them that should not be a big issue but extra room always comes in handy when you integrate. Roost space is an important part of that extra room.