1) Can they fly? Can you clip their wings or do you need a roof over your pen?
Depends on the size/breed. Mallard types, call ducks, and wild exotics can fly quite well. The heavier they are, the more land bound they are. Yes, you can clip the flight feathers. You want a roof over their pen to keep predators out... you know the term "Like sitting ducks"... a large enough owl can carry off a surprisingly large duck.
2) Are they friendly if handled when young?
They start out friendly, then go through a "I hate humans" phase, can last a week or much longer. They settle back into being pets with treats and talking to them.
3) What kind of housing do they need? How much room? Do they need water?
They cannot "hop" into a house, and a steep ramp they can't navigate that well either (as far as typical domestic ducks) so something low to the ground is best with steps or a short ramp. A large type duck should have 4sq ft minimum in the house. Water inside is VERY messy unless you have a way to collect it. You can have a portion of the floor as hardware cloth, so that waste water falls through it and leaves the bedding dry. Or you can elevate it and put a catch pan under it, also covered in wire so that they can't play in it. If they have food, they must also have water. If you can be reliable in letting them out early, they would be fine over night without after they are adults. The babies will need full time access.
4) Do they have a breeding season where they lay or do they lay year round?
Depends on type. Some are seasonal layers, some are more production minded and lay 200+ eggs per year.
5) Can male ducks be kept together?
Depends... though usually they'll fight during the breeding season if you have them closely confined or with less than 3 females each. Some can get REALLY mean. If you're going to let a female brood on a nest, you need to remove the male. They might kill the babies.. or help out in rearing... but better safe than sorry. They may get along if there are no females around at all. But they might not. Hormones are hormones.
6) Can you keep chickens and duck together?
Depends on your set-up. Ducks are messy and go through water like you wouldn't believe, leaving the chickens with dirty water. Chickens don't like the damp, and ducks are very damp. So if you can keep things dry through water management, and the chickens have access to fresh clean water out of reach from the ducks, then yes you can. Having chickens around to stir up the bedding is good too, since ducks flatten it out and make a mess of it. You'll need a poo board under the roosts, so that if the ducks sleep below the roosts, they don't get pooped on.
Ducks are VERY messy. VERY MESSY.
But they're hilarious and it's worth the extra effort. If you have them on dirt it will turn to mud and stink to high heaven. If you're going to pen them up, put down pea gravel. Sand will stink when it's wet. Gravel, they can get it as wet as they want to, and you can hose it off for cleaning. They dig holes in the mud and get filthy. They're poop machines. They mix their food with water and make a yucky mash that they sling everywhere. I have mine on the gravel where I can hose it away, or on a wire covered feeding station where the waste falls into a tote I can remove and dump.
Did I mention that they're messy? They're very messy.