thanks for the help...it was ages i had not play 6thOk I cracked open my 6ed book and looked. So yes, the rule specifically says that a character with one or more blessed spawning can join any unit.
Additionally a unit with one or two blessed spawnings can only be joined by a character with the same spawnings. (Note that it doesn't say "the exact same spawnings")
So in your case with the Sotek and Quetzl spawning, I agree that you are legal as the character still has all the spawnings as the unit.