Constellations lore

[iiroro]

7 Dragons, 7 Constelations, 7 Eorzian Elements.

The basis for the last three being Tiamat js ocationally the Arcfiend of Water, Lightning Crystals apear near where Ratatoskr was killed, and Bahamut being traditionally non elemental. Thus leaving only the Earth Dragon unacounted for.

For the 7 Constellations, or the Heavens, there are also 7 corresponding Hells.

As for your speculation with the dragons, you have forgotten Ice. I agree that Hraesvelgr is possibly associated with Wind, as in Norse mythos he is a giant who shape shifts into an eagle, but he's elemental affinity may instead be Ice. My reasoning is mostly because his association with Saint Shiva (Shiva always being represented as the Ice Summon in the FF franchise), and to a superficial extent he's white...

I'm also not sure if Ratatoskr's elemental affinity is Lightning, again going back to the Norse mythos, Ratatoskr is a squirrel and the only squirrels I know with Lightning powers are in Smite and Pokémon.