To answer your questions:
Thy should have a coop to sleep in at night time, otherwise they're vulnerable to nocturnal predators.
Guineas are vulnerable to the same kind of predators that other poultry are. The main difference is that in a large enough group they will become rather aggressive. I've seen mine pick fights with hawks, vultures, wild turkeys, coyotes, dogs, deer, and a forklift.
I'm not sure what you're asking about a being ina coop all winter. If anything, they're more cold-hardy than my chickens. They're a little weird about snow, but so are.chickrns that haven't seen it before.
As far as "nice" or "aggressive" goes, guineas are just different. They're more feral than most poultry and they have their own weird little quirks.
Compared to chickens, they tend to be more along with people and way more pushy when it comes to other birds. If you have chickens, how well they get along depends on how well your chickens accept the fact that the guineas are in charge.
Guineas are a lot more flock-oriented than. Chickens, and they prefer to be on larger groups. If left alone they actually panic and start acting kind of crazy.
If they get used to you, they'll actually be quite affectionate on their own weird way. Mine will greet me with their strange little clicks and chirps if they haven't seen me in a while, and if I'm outside doing chores they'll check me out and even and out with me.
On the other hand, they're extremely territorial and xenophobic. They don't kike strangers of any kind, even other guineas. They'll scream at trespassers, and try to bully and chase them off whenever possible. This may get them killed, and it will certainly annoy any neighbors you have. Mine took a disliking to a neighbor's Rottweiler and tried to drive him out of his yard repeatedly. I ended up having to move the fence line back about 50 feet so everyone could have some peace.
Overall they're pretty awesome birds, but they're definitely not for everyone.