When we first got guineas, my husband thought like yours. After the trauma of losing a lot of birds in a matter of minutes, they now have the Fort Knox of runs/coop. So let's back up..
Most chickens & keets are sold at hatch. Not sure what your arrangement is, but that means a whole lot of babies in basically a shoe box. Have you special arrangements that you're getting birds >4 wks old? Otherwise, you'll
a. Have plenty of room in your car, and
b. Need to have them in a heated brooder for 4-6 weeks,depending on your, them, the temp outside, etc.
After that, yes, they need to be shut in their coop to learn this is home & to return there every evening. I'm a bit more hands on & home with them, so mine get to come out during the day with me while I'm out, and hustled back in when I'm done being out with them regardless of the time. But they have to learn to be inside before sundown.
They will not know to stay near the barn,nor have anyway to defend themselves from your bobcat, a raccoon, a hawk, an owl, a passing car,a dog, etc etc etc.
Guinea fowl have mono vision (see out of ine eye at a time), & can't see in the dark. They are easy prey in trees at night, bc they can't see, so can't fly.. An owl's flight is silent & deadly. If a raccoon is in the tree, the most a guinea can do is side step on the branch before being attacked.
Watch a few videos of guineas chasing & fighting-that's how they "play". Chickens will never understand that, & end up hurt, dead or neurotic if housed together.
Try to get more now..,so much easier than trying to integrate with older guineas later.