Ascians

[CyrilLucifer]

I wouldn't necessarily say that we don't have anything to verify the theory that Xth represents the Ascian they serve. Chalice's dialogue has lots of watery puns, despite Lahabrea being fire-based in XIV. With higher servants not matching the master's element, but rather his "trigon", that absolutely puts 12th under Pisces, Cancer, or Scorpio as well for reasons I edited into my post up there. It doesn't completely cement it, but it does add to its defense.

Edit: And this is, of course, ignoring that they openly say they serve Lahabrea.

[Cilia]

I wouldn't necessarily say that we don't have anything to verify the theory that Xth represents the Ascian they serve. Chalice's dialogue has lots of watery puns, despite Lahabrea being fire-based. With the servants not matching the master's element, but rather his "trigon", that absolutely puts 12th under Pisces, Cancer, or Scorpio as well for reasons I edited into my post up there.

Since when has Lahabrea been fire-based?

In the Ivalice mythos, Mateus is the ice esper. (Water is a very related element.) During the battle with the Ascian Prime, it uses both Ice and Fire moves; you could say the Fire is from Igeyorhm (who has the poorly defined element of "soul" in Ivalice mythos), while the Ice fits right in with Lahabrea's connection to Mateus. Additionally, as you pointed out, Pisces is one of the three Water-elemental Zodiac Signs, so the Ascian of the Twelfth Chalice making water puns is to be expected. Additionally, given the Chalice is the Water-aligned suit of the Minor Arcana (which the lesser Ascians seem to take their names from), this only makes more sense.

[CyrilLucifer]

It seems to work differently in XIV than Ivalice. Gameplay-wise, Lahabrea was definitely, absolutely sticking to using Dark and Fire elemental magic in the Research Facility. Same with Igeyorhm using Dark and Ice and Nabriales with Dark and Earth in their battles. That was basically the entire point of the two working together was that their elements in-battle contrasted. That's why you attack Igeyorhm first: her freezing can be really, really annoying if you don't.

It was a really weird moment for me seeing him having that focus on fire because, logically based on Ivalice lore and astrology, he shouldn't have been. It also shows that we can't follow Ivalice completely, despite it being a backbone.