While we aren’t that far apart, I think you normally get more severe weather than I do. The cold isn’t your problem in winter, you get some snow. Maybe not a lot, but enough. I think that’s when you space issues may show up. When my chickens wake up to a new white world they don’t want to go out in it for a day or two. They don’t like a really cold strong wind either. They pretty much confine themselves to the coop. Even if your run is covered, snow blows in from the sides, as does rain. That roof might not provide much shade in summer either, sunlight also comes in from the sides. For various reasons you might want to put some protection on a side or two as well as on top, at least in season.
I personally don’t believe in magic numbers for chicken space. Many people believe that 4 and 10 are just magical, but if you follow the link in my signature you might see some reasons I don’t believe it.
Chickens do not understand the concept of coop space versus run space. They understand the concept of having space available when they need it, wherever that is. Since you never lock yours in that “coop” section, the entire area can be considered “coop”. The exception to that might be when snow or a cold wind limits them to the coop section. That’s why you might want to put some side protection on that, at least in certain seasons.
I personally believe in providing as much room as you reasonably can. I find if I overcrowd them (and sometimes I do) I have to deal with more behavioral problems, work harder, and have less flexibility to deal with issues when they crop up. To me that flexibility is tremendously important, it can cause me stress problems if I don’t have it when I need it. As long as you have no more than one rooster, don’t have a broody hen hatch and raise chicks with the flock, don’t try to integrate chickens in the future, and you can make most of that run available in the winter I think that can handle six chickens fine from a behavioral aspect. You are dealing with living animals so no one can give you any guarantees on behavior, but I would not worry about that.
The more you crowd them the more the poop builds up in a limited space. You may have to work harder than I like in managing that poop. You may not have much space to put in a broody buster or a place to isolate a chicken from the flock if you need to. The less space you have the harder it can be on you. But yes, I think you can make that work for six chickens in central Oklahoma.
The issue I’d have with a duck in there is that ducks need water and generally make a wet mess. Your “run” will have poop in it, chickens and ducks poop a lot wherever they happen to be. When poop gets wet and stays wet it stinks. Wet poop is unhealthy. Totally forgetting about space issues, if I felt I needed a duck, I’d strongly suggest you get two and build a totally separate facility. While some people manage, keeping ducks and chickens together usually doesn’t work out well.