1. I think coturnix are pretty hardy. but I've only had coturnix, so...
2. most say at least 3 hens and one roo, but some roos are picky and will keep making noise even if you have the right ratio. I also had a gentle roo who was satisfied with one hen and didn't pull her feathers and only gave her a tuft on the back of her head.
3. I give them
Coturnix quail is quite cold hardy as long as they stay dry and have shelter.
For coturnix quail, you want 1 roo to 3-5 hens.
Game bird starter is best for chicks (around 28% protein). You can switch to a layer feed when they're adults.
Technically Quails don’t have breeds like chickens, they’re mutations from the one species. I’m not entirely sure which one you’d want but probably Japanese/Coturnix or King/Button because they’re the most common. They’re all ‘cold hardy’ if they’re dry and warm. It also depends on the daylight hours and the weather where you are. Some places high or low on the earth (Canada, Tas, Aus) will obviously get cooler than places like Nevada, Northern Aus. The boy to girl ratio depends on the quails themselves. King/Button quails are really good in pairs and I wouldn’t recommend otherwise. Japanese/Coturnix quails are better with 1 male and about 3, 4 or more can be recommended to avoid overbreeding but I know people with trios that are fine and even pairs but for you if you’re wanting Japanese/Coturnix then I’d recommend more of the higher ratios with 1 male to ..... many females. With food I think everyone else has covered it well. I give my quails gamebird finisher when older and starter when young as well as other things that help with calcium which people have said.