Merely looking at the Ivalician Summons in my list, if you remove Zodiark from the list, there are exactly 12 Summons. So if Zodiark were to be like the "Dark Hydaelyn", then it is feasible that the others would be kinda like "The Dark Twelve" or something. We already know that Zodiark and Belias exist. Belias is an Elder Primal, which puts it on the same level as Bahamut in power, and Zodiark itself is supposed to be as powerful as Hydaelyn.