Correct me if I'm wrong; A warcroll must have the SERAPHON Keyword to be able to benefit from the Seraphon allegiance ability, or some sort of battallion must provide the needed keyword. Otherwise it goes into the allies pool. In this particular case, the Skink chief does't have the SERAPHON keyword, and since the only allies Seraphon can take are the STORMCAST ETERNALS, you'd have to run a grand alliance order list just to be able to have him in.
That makes sense. So then the next question. Why even have the Lizardmen warscrolls at all? I can't see any use for them if you can't use them in a Seraphon army.