Mille Fleur is often used to refer to a particular color of d'Uccle. Mille Fleur is the color, d'Uccle is the breed. D'Uccles are feather footed, d'Anvers are clean legged. Both have puffy cheeks & can have the Mille Fleur coloring. It will be rather difficult to tell if this particular baby is pure or not since the breeds are very similar other than the leg feathers, but it definitely has d'Uccle in it since the legs are feathered as long as the d'Uccles are the only feather legged breed you have. Nearly any chicken can interbreed with any other (as long as they are at least similar in size). You would have trouble getting something like a Serama to cross with a Jersey Giant however.