Is there anything that addresses whether dragons can launch exploding spheres of their energy type instead of the standard cone/line?
There was one to change cones to lines and vice versa. I don't recall one that let you do your desire. That being said, as long as you keep it balanced to the PCs I don't see why as a GM you can't do it.
Fire Drakes have a fireball breath weapon.
Flyby attack cast fireball.
