Galactus is powerful enough to defeat a single Celestial, if was attacked by the Celestials I can see Earth X prt 2, Galactus can take out a lot of Celestials out before going down.
Galactus would be beat Tiamut, people read too much into Galactus "now knows fear", when Tiamut awoke, in the context it could mean anything because we didnt know Tiamut true intentions at that time!
Based on powers of Galactus and what he can do, it's very possible Galactus could take on any single Celetstials or a host and defeat them if he had too.