For my size coop how Many Chickens/Guineas do you think I could fit safely?
Well, I think the recommendation for a full grown chicken is 3 to 4 sq ft of floor space per bird. I personally like to give them much more room if penned. Just more sanitary the more room they have. So it looks like for 12 birds you have just the minimum space needed. You can always attach a ground pen to the side of the coop though. Just some posts and wrapped in chicken wire should be sufficient for daytime use. Just make sure its either tall enough so they can't fly over or you net or cover the top. As long as the little access door under the coop is closed at night they should be safe from nighttime predators. Even though you plan to free range you will find there are times you may need them penned. That's a very nice coop by the way.
Oh, one more thing. I don't know if you know or if it had been discussed elsewhere, but when you bring them home, you might keep them penned maybe about a week so they will know where to go back and roost at night. If you turn them loose the first few days, they may end up roosting in the trees.