I don't remember Bahamut that well, but wasn't Kujata standing on Bahamut as it swam the oceans?

Tiamat is the female incarnation of Apsu. This is from one of the Mesopotamian societies. I know it isn't from the Persians, because they incorporate the Zoroaster with Ahura Mazda and Ahriman. Tiamat and Apsu are either Sumerian or Babylonian. I think it would be pretty neat if we could so Apsu as a summon. The only video game I've ever seen Apsu in was Sailor Moon: Another Story on the SNES.

I agree that Odin is inconsistent. For once I'd like to see him appear in a game that isn't FF Legend 2 with Sleipnir, Gungnir, and the crows. FF Legend 2 had a pretty neat idea when it sent you to Valhalla after you died in battle. From there, you could speak to Odin to continue your game.

Alexander I don't get either. Perhaps it is a reference to Alexander's kingdom of Alexandria, an iron kingdom?

I've never heard Knights of the Round in reference to the Last Supper before. I do know that I have seen many stories in which there actually are only 12-13 knights of Arthur's round table. It may not be accurate, but there are so many versions of that legend floating around that anything is possible.

I thought FF8 got pretty creative with the GFs. Quetzelcoatl in place of Ramuh, who took the place of Indra. Indra doesn't ring an immediate bell, but I'm pretty sure it is connecting with Hinduism and has some relation to Agni and Dagon (also seen in Phantasy Star Online).

Pandemonium was a cool one. It is a shame they had to waste such a unique reference on a summon that was completely worthless.