I started with 7 guineas last summer. They were in a coop until they were almost full-grown, then i let them out and they free-range 24/7. I would be happy letting them stay in the coop with the chickens, but they beat them up. I also want them to be able to free-range all day and eat bugs this spring. I don't let my chickens out until afternoon.
The guineas roost in a tree above the chicken coop. I've gradually lost them one at a time over the winter. I didn't lose a single one until the first freeze. Then, they slowly would disappear. I'd find feathers occasionally. Sometimes, I find nothing and I wonder if the guinea will show up again one day. I'm down to 2 guineas. Since I lost the last one, I haven't heard the buckwheat call, so I guess I only have males now. They really like to hang out with my chickens and I've contemplated leaving them together in the coop to see what happens. But, I don't want to leave them in the coop all day so that probably won't happen.
As for your question, they will definitely roost in the trees. One night I couldn't find my guineas in their regular tree. I searched and searched for them, and I finally found them by listening for their chirring noise they make at night. They were up about 30 feet in the top of two separate trees.
I have no experience integrating a young guinea with older ones.