Not all Guineas flocks are the same or behave/act the same, and not everyone's experiences with their flocks are always going to be identical, so some of what you have read can be entirely true. I always try to keep that in mind when I am giving Guinea advise, but some are oblivious and think their birds' behavior is typical of all Guineas... in my experience that something that is definitely is not always true.
My Guineas are very effective at pest control, especially the ticks, grasshoppers snakes... and the big hairy wolf spiders. Pest control is why I got them in the first place and they have definitely earned their keep in that department. They are also very effective weed and weed seed munching machines. But since I always feed my breeding flocks greens veggies and tomatoes from my gardens, I do not allow any of my birds in the gardens at all... they would strip my veggies clean in a heartbeat because they have been fed those plants from the time they were keets.... so I cannot attest for their effectiveness of eradicating japenese beetles from a garden.
I have a lot of Guineas (light colors and dark colors), but not a high hawk threat, (I do not own chickens)... I have never lost a Guinea to a hawk in 8 yrs. My Guineas will sound off the alarm call and run for cover if any large bird flies overhead tho. I think the more Guineas you have the more alert they can be to hawks and other skyward dangers, but since I've always had a lot of birds I cannot prove that, just makes sense for my multiple flocks. If one of my flocks sounds off because they see danger then they all sound off, look for the danger and will run for cover if needed.
As far as roosting with your chickens/sharing a coop... that could eventually be a problem, especially during breeding season. Males with raging hormones typically become overly aggressive in close quarters. BUT, that being said some people have perfectly happy mixed flocks, some have nothing but problems... I do have several friends and keet/Guinea customers that have happily mixed flocks, so it really all depends on the particular birds and the coop/run set up and poultry routine. Guineas need a lot of space and free range time to stay content tho, and more than just a few other Guineas in their flock to keep their focus on their own kind. If you have a large coop and everybody gets plenty of free range time that can be the key to a peaceful mixed flock, (but it doesn't always guarantee it). Laying Hens can become too stressed out from the aggression level in the coop and pen, and their laying can suffer from it.
Best case scenario (IMO) would be to have a separate coop/pen for each flock and just free range them all together for the hawk warning/alarm call benefits.