I am just wondering since it doesn't have any requirements pertaining on using breathing style techniques you just only need to learn how to use it with proper training, Does it mean it is possible for a demon to use it?
Another demon who uses a breath style is,
Kaigaku "Kaigaku (獪かい岳がく Kaigaku?) was a Demon Slayer in the Demon Slaying Corps, and Zenitsu Agatsuma's senior disciple under Jigoro Kuwajima. However, he became a demon to save his own life and became Upper Moon Six of the Twelve Demon Moons, replacing Daki and Gyutaro. " Kaigaku .
He uses thunder breath style.