I'm almost as new at all this as it gets, but I've been studyin' quite a bit ... I've 15 keets and 26 chicks in a divided brooder. The keets are far too large, in my case, to allow to be with these chicks just yet, and they need time to get a bit used to one another -- slow introductions reduce some of those other issues to a minimum, which is very important when they grow up -- guineas can be very dangerous, given the right (or, wrong, for the loser) circumstances.
I'm no fan of completely free-ranging chickens, as they aren't nearly so kind to your neighbor's prized flower beds and new cars as the guineas will be.
If there is such a limited danger, most esp. during the daytime, I'd suggest a tall fence (mine's six foot) around the coop, with landing boards on top. The chickens can't get out without an open invitation, but the guineas will fly up 'n over whenever they wish to ... they'll return at night, so long as they're happy with the roosts you provide, and you confine them in that area long enough for them to consider it home (I have a large tarp, and chicken netting, which I'll use over the chainlink runs our area requires -- in my case, I'm gonna give 'em their own exit to 'fly the coop' to the outside world, but in a way that the chickens shouldn't be able to navigate from inside, and predators shouldn't be able to easily enter through (still workin' on that part ~'-)
I've seen many suggestions, in regard to sq.ft./bird, and there's obviously other factors to consider in any such equation. With good planning, and highly efficient use of what space you have, you should be able to crowd 'em up a bit more than another's configuration would allow (i.e. raising the roof of the coop much higher than one normally would, but leaving a large amount of space between the highest/larger roosts closest to the roof/wall, and having something to keep the droppings from landing on those lower ones that the chickens would use).
Also, the guineas will most likely never use the chicken's nesting boxes to lay eggs in, even if you didn't have any chickens ... so, you could probably reduce them in number/size, so as to both recover the cubic footage wasted and discourage the guineas from even entering them.
I tend to over-think, over-plan, etc., but sure sounds like you're headed down a road similar to my own (but, that yours has a much smoother surface ~'-)
