Ixidron basically says all face-down cards are creatures. Is creature a type that Coat of Arms would be pairing with and make everything huge, or would coat of arms essentially do nothing to all of those face down cards?
From that same page (emphasis mine):
Creatures turned face down by Ixidron are 2/2 creatures with no text, no name, no subtypes, no expansion symbol, and no mana cost.
"Creature type" is a shorthand for a creature-specific subtype (205.3m in the Comprehensive Rules). Face-down creatures have no creature type, so Coat of Arms does not affect them.