There really is not definitive answer for this one as it depends on a few factors. Coryza, along with a few other bacterias, are the most contagious and deadly to quail. Coryza is a respiratory ailment, and is THE most deadly. It is carried on the air by breathing, sneezing or through drinking water or feed dishes. There are a few other bacterias that are harmful to quail passed on from chickens that are carried thru the chicken dander, which as you know, chickens are very dusty, and of course picking up poop on your feet. Your hands as well will carry all diseases around and spread them to your quail
So to get back to the question of what is a safe distance apart you can safely keep quail and chickens...You would never want to keep the chickens up wind from quail. Definitely never close enough for contact. Bio security must be practiced diligently...washing your hands between the two species, change your shoes if you keep quail in a walk in aviary style set up and of course never share feed or water fonts, or any cleaning tools. How far does dander, breath and sneezing travel? I have kept chickens and quail 20 feet apart with no ill effects. But I would never keep them closer than this. Some people have successfully kept chickens and quail together. But it is never worth the risk to your chickens or your quail.
And you have to remember, quail can be carrying virus's or bacteria that chickens can contract as well. Not as common, but it does happen. Your chickens have become immune to their environment and everything in it. When you introduce some new bacteria, even in small amounts, their immune systems now have to grow immunity to the new pathogen.
So, if this was my set up, I would not keep quail above my chickens. Not only would I be risking the quail getting sick and dying, but I would not want to expose my entire flock of chickens to what ever these quail may be carrying. If i did not have enough room to keep quail separate, then I would not get into quail if I already had chickens.
Poultry do not have to be exhibiting any symptoms of anything to be carriers of disease.
All this being said...Welcome to BYC and the quail forums!