I've never seen any mention about any tribe that wasn't the U one. What I do know is that there isn't supposed to be an unique tribe per letter. The lore is made in a way that leaves room for personal interpretation, since there are several sub tribes of each letter, and each of them will have their own rules (so long as the basics are respected). We had freedom to decide about our K tribe, so I think you could do the same ^^
Clover Blake (Hyur) /Â K'mih Yohko (Miqo'te)