Since you have an ideal climate for figs, I would space similar to typical orchards if you have the space- maybe 16-20' in either direction. Even here in zone 7b, we have figs 25 feet wide. Of course you could prune to be smaller, but I'm not sure if that's really time efficient if you have a lot of trees. Your CA soil is probably pretty great- maybe just add manure/nitrogen. I would primarily concentrate on a large amount of mulch for each tree. That will conserve water tremendously and each year add nutrition to the soil. We're big on biochar here, but you may not be able to burn there in CA. My father had to pay a small fortune to get agricultural waste hauled off in dumpsters because he couldn't burn it there in central CA. The best to your endeavors.