Well, here're my thoughts.

A cross clan child is likly possible such as a Miqo'te who is both of the Moonkeeper and Sunseeker lines, or an Eelzen who is half duskwhite andnhalf Wildwood.

But the "races" of Final Fantasy XIV are actually different species. Our world's version of Racisim would actually be tribalisim in Eorzea like when a Wildwood looks down on a duskwhite or even, say, looking down on a Sunseeker because "All sun seekers are scum" would be an example.

But as for cross species, so far the only cross species compatibilities we've seen are Hilda her self who is not only The Only Hyur-Elezen we've seen, but the only confirmed cross species NPC in the game.

I personally view that as being sufficient enough to avoid the "Roelala" paradox being that, due to genetic similarities, Hyur and Elezen are THE ONLY cross compatable species in Eorzea . (We still don't know enough about garlen pure bloods to consider them yet)

This theory is kinda based on how real world cross species can produce a cross species animal such as a Liger (Lion-Tiger Cross) or Mule (Horse-Donkey) but thinks like a Fox and a Wolf cant. (Because Vulpins and Lupines are not genetically simmilar)