I am beyond thrilled to see so much interest in my beloved NNs! I wholeheartedly believe that for our hostile desert environment they are an ideal breed. Now, there are a lot of points I want to address, so please bear with me....
1) A/C in chicken coops: I would say the ultimate goal is always adaptability in your birds. It certainly is in mine, but I will confess to having two window A/C units to use as necessary in the worst part of summer. One of my chicken coops is actually a cabin I think would have made a fantastic starter home for my husband and myself when we were starting out many, many....many years ago. This is the cabin before I divided the inside up into separate coops, added pop doors and outdoor enclosed runs, etc.
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The two windows on the front porch are where I put the A/C units in June. Why? Because not all of my chickens are NNs! When I first began keeping chickens I started with hatchery birds to "get my feet wet" and learn how much work it would be. Then I fell in love with the idea of having some Bielefelders, which are wonderful, wonderful birds....and not at all suited for this environment. The ONLY birds I've lost to heat have been the Bielefelders, and while I still have two hens (not very good layers but exceptionally sweet), and my favorite rooster of all-time, summer is VERY hard on them. My husband and I bought the A/C units to create a cool area for them outside so they could stop living in my master bathroom shower on hot summer days.
So, do you need A/C? That depends upon whether you want to work with the nature and the genetics of the birds, or thumb your nose at it and have what you want at any cost. I should note, however, that even my Biels have slooooowly adapted somewhat to the heat and I didn't use the A/C units nearly as much this year as I did the year before.
2) Coops:
@cactusrota made an EXCEPTIONALLY good point. Open air coops with lots of shade are far better here than "traditional" coops that keep the birds all snug (and hot) inside. Here's an image of one I found on CraigsList that's similar to my own favorite:
https://phoenix.craigslist.org/evl/grd/d/chicken-coop/6299868253.html
Notice that the back wall and roof are solid, but most of the coop is very airy and open. This is a very good thing out here, even during our brief cold spells. I'm a firm believer that more air is better for chicken health. After all, they're birds, and they evolved to live in the wild.
3) Mixing NNs with other fully feathered breeds - YES! Do it! There's no problem at all. I've been doing it for years, and cross-breeding my NNs with fully feathered varieties to improve on certain traits I'm looking for, especially heat tolerance. My NN x Bielefelders are not only my sweetest hens, but their egg laying improved during the summer months (X-Lg eggs!) and their heat tolerance improved exponentially. While many of my fully feathered breeds slowed dramatically during the summer, the girls in my NN egg-layer focused flock continued to each give me 4-6 eggs per week, weighing 2 ounces or more each (and in a variety of colors).
And if you are looking for good, slower-growing meat birds, NNs are definitely your breed. Having 50% fewer feathers makes processing a lot faster, and their skin crisps up like no other bird I've eaten. (Yes, I do eat many of my birds.)
4) NN Personalities: In my years of keeping NNs I've had one truly aggressive NN rooster who absolutely hated my husband and son (but not me), and he was in my starter flock. Now, granted, I've worked very hard to breed out aggression in my NNs, but it honestly wasn't all that hard. Even the offspring of my lone aggressive rooster came out much more human friendly than their daddy was. Nearly all of my best, most attentive roosters have been NNs, and I have multiple roosters living together without fear of any real problems. Yes, the boys spat from time to time, but usually only during adolescence and to establish pecking order once the young'uns are released into the yard to interact with the older birds.
NNs are smart, very hardy, affectionate, friendly and quirky. Only my NNs and my Australorps have proven to be good hunters (lizards, mice and bugs), feed hardy, and super healthy. And personally I love that they can all look completely unique from one another or you can breed to SOP. Me? I love the eye candy...and playing with the color genetics. It allows me to focus on them as my primary breed while also allowing visual variety in appearance.
Oh...and I live south of Tucson in Green Valley, so feel free to message me with any chicken questions you may have. I've helped several people around here get started with chickens or improve upon their setup and had some lovely visits with them to boot.