The Fae in all their various breeds share the same taxonomic kingdom, phylum, class, and order as humans with only the difference being their genus which is simply labeled Fae. All Fae share this same genus regardless of other distinctions. For example, a Sucubuss, would be Fae succubi. All Fae appear to be extremely long-lived compared to humans. According to McCorrigan, Fae do not allow divorce for the first thousand years of a marriage in order to "give it a chance." (Fae-nted Love) McCorrigan himself is in fact centuries old despite his appearance as are Zoie,Ernie,Ishild, the Norn and many others. This is because young Fae age and develop at nearly the same rate as humans until puberty after which they are physically matured and thus no longer age as humans would. It is unclear whether or not they stop aging all together or whether they simply begin to age much more slowly, though some Fae have been known to die of old age. How their Fae abilities affect their aging is also unknown.
Although different Fae spieces can apparently interbreed with each other, it is unclear how Fae children's powers are inherited. While Geraldine is a succubus, her father could have been an icubuss before she learned that her mother was a succubus (Her father's true identity remains unknown). The Wolf-shifter Daire once claimed that his grandfather was a Undie a male water spirit who reseaves a human soul by falling in love, the element being of water, (although he may have been merely joking given the context when he revealed the information).
McCorrigan and his sister Chinonso Nima, are different species; Brady is a siren, like his mother while, Chinonso Nima is Pombéro, although it's not stated what their father is. It has also been revealed that hybrids of both Fae parents are possible, such as the glaive, possessing the power of telepathy from her father (telepath) and control of some aspects of nature from her mother (Wood Nymth) Geraldine and her mother Saskia are part Blood Sage and their blood can enthrall Fae and humans.
Fae do not appear to be very prolific compared to humans. For example, Sturgis is thousands of years old but has only two children, neither of whom have children. A human a century old would likely have several generations of descendents. Their immortality may simply mean that they have less incentive to breed where humans are encouraged to do so.
Fae and humans can also cross-breed, but the resulting children will not inherit Fae powers, and Fae are discouraged from having contact with their human children.