Is Galactus a celestial from the previous universe?
The Celestials origins are unknown, I believe its speculated that eternity might of created them, but there have been scenes of them being born from dead universes iirc, its unknown, and galactus in the previous universe was just a scientist i think, who was normal human
I thought the most powerful Celestial was called "The One Above All"(not TOAA the Tribunal always talks about). As for the fight it self, I think a fully fed Galactus would be able to beat anyone of them in a one on one fight but if they were to all team up on him I think he would be overwhelm.
TOAA might be their leader, but Exitar's the guy they call when some celestial need to be "disciplined" (like the dreaming celestial) or when there's dirty job to be done.
Just the fact Exitar's about 10 times bigger than the other celestials serves as an indication.
And the Invisible Woman was still able to hurt him. Also Celestials are born from dead galaxies with the most recent being made from the Black Galaxy which was a galaxy that had all it's star's snuffed out by Ego.