H-heh...another excuse to bring up Garlemald in a thread!
There's an NPC called Arenvald in The Waking Sands. At one point it is revealed through flavour text that he's a Garlean half-breed. He uses the Highlander model and may or may not have a third eye beneath his hair that conveniently covers his forehead. So Garleans can seemingly breed with other types of Hyur - which isn't too surprising - but I suspect Garlean half-breeds are shunned by many based on politics and a fear of them boasting conflicted loyalties.