I wouldn't advise mixing Quail with Chickens,although no doubt someone, somewhere has succeeded in doing so! The two species would probably come into conflict if they were kept together in an enclosed space. Quail can be pugnacious, and most breeds of chicken, even the smallest bantams, have a size advantage over Coturnix Quail at least!
The smallest breeds available come from the group called True Bantams. these are breeds of chicken with no large version. The smallest of these is undoubtedly the Serama, a malaysian breed, readily available in the UK, that comes in several different size categories ( all of them tiny! ) and a rainbow of colours and patterns.
Other breeds that follow close on the Serama's heels in terms of size are Dutch Bantams, Modern Game Bantams,Japanese bantams, Belgian Bearded Bantams, Sebrights, Rosecombs and Pekins.