I'm about a year into my mixed-flock "experiment" and so far it's gone pretty well.
I managed to get them to live together in about as much harmony as poultry do, get the guineas to go into the coop every night, and get the guineas to (mostly) lay eggs in the nesting boxes.
One caveat: I am not an expert on guineas, so none of this should be taken as "advice." It's just some observations about my own mixed-flock experience, and how I got where I am now, which may or may not be of use to you:
1) Guineas are very "flock minded." If one gets isolated for even a minute or two they panic and literally lose their minds: they'll forget where they door they've used their whole lives is and pace back & forth trying to walk through the barrier between them and the rest of the flock.
Opinions vary on the exact number, but you need at least 5 to 10 guineas for them to behave normally. Otherwise they get stressed out and over-aggressive.
2) I read in several places that guineas don't like going into dark places, and I've observed on average their night vision is bad even for poultry. I have two that are completely blind after dark. I designed my coops with white interiors & exteriors (in part to help reflect the heat of a Georgia summer) and the coops & runs have motion-activated lights to help them find their way in at night.
3) Guineas are semi-feral and need to be "trained" that the coop is "home." Otherwise they prefer to roost in trees, which makes them easy targets for owls. In my case I moved them out to the coop at 2 weeks old. I kept them "cooped up" for at least 4 weeks, and when I did start free-ranging them it was only for an hour or two before sunset and only a part of the flock at a time.
4) I had some issues even then, mostly the two males that chase each other constantly. One would chase the other on top of the run. The solution was I'd wait until the "aggressive" bird went to roost, then I pushed the other one off the top of the run and he'd walk right inside. Earlier I put some plastic grid over a muddy area in front of the run entrance, and about a third of the guineas hated it so much they wouldn't even jump over it and instead tried to roost on the coops & run. Removing the offending ground cover stopped that.
5) I'm of the opinion guineas need to be free-ranged. The times I've had to put them on "flockdown" (most recently after a string of bobcat attacks last month) are when they've behaved their worst.
6) Getting them to use the nesting boxes was a combination of patience and luck. Initially the guineas were total chaos about laying eggs: half the time it looked like they'd just squat whenever they got the urge and dropped the egg wherever. I'd find them randomly in the woods, the middle of the lawn, just outside a nesting box, etc.
Then they started finding nesting spots in the woods, which either I'd find and remove all the eggs, or I'd find the remains of an egg buffet some forest creature enjoyed. Sometimes they'd follow a chicken into a nesting box and lay with them (sometimes literally, as in sitting in the same box with the chicken).
When some of the chickens went broody, I used them to hatch some fertilized guinea eggs. This got the guineas much more curious about the nesting boxes. I also started using fake eggs as a way to trick the guineas into thinking I wasn't taking the eggs away (they don't seem to really be able to count).
I also make it a point not to disturb a nesting guinea. My chickens I can stick my hand under them to take or inspect the eggs and all they'll usually do is make those weird cat-growl noises they make. A guinea will shoot out of there the minute the door is wide open, often not to return that day. So I let laying guineas alone.
