Half breeds are canon, just very rare, according to a Dev post a loooong while back. I'll go see if I can dig it up. Clan mixing is also canon too. (F'lhaminn, Minfillia's foster mother is mixed clan according to Fernehalwes.) I'll edit this post with the link once I find it.
EDIT: Race Mixing | Clan Mixing
FOUND IT! Finally.
EDIT: Race Mixing | Clan Mixing
FOUND IT! Finally.