I do not know anything of FF-6-15-150
USDA 6-15-150 is a hybrid of Lee Mandarin and Orlando Tangelo.
That makes it a (Clementine x (Dancy x Duncan Grapefruit)) x (Dancy x Duncan Grapefruit) cross.
Fruits are the size of a medium mandarin, but with an orange-like exterior texture
Flavor is described as flavorful, lovely subtle flavor, tangy undertones, very juicy, difficult to section, seedy
was said to be the most cold tolerant mandarin to come out of the USDA citrus breeding program in Florida, on par with Satsuma mandarins
poncirus hybrid US119 that is in its pedigree is not more hardy than navel oranges.
supposedly hardy to 10°F, young trees injured in low 20s