If an alien of a certain species is shown (but never named, or does anything) in Star Wars movie, and then given a name/backstory/whatnot in a C-level book, is that alien species (and the facts about it) considered a G-level canon or a C-level canon?
Or is it nuanced? (e.g. the existance is G-level but any details are still C-level)?
Everything that is explicitly stated in a G-Canon source is G-Canon and nothing else, the rest lies in a lower level of canon. A good example is the group of bounty hunters Darth Vader mandates to find the Millennium Falcon.
They are all named in the Script
The group standing before Vader is a bizarre array of galactic fortune hunters: There is Bossk, a slimy, tentacled monster with two huge, bloodshot eyes in a soft baggy face; Zuckuss and Dengar, two battle-scarred, mangy human types; IG-88, a battered, tarnished chrome war droid; and Boba Fett, a man in a weapon-covered armored space suit.
VADER: ...there will be a substantial reward for the one who finds the Millennium Falcon. You are free to use any methods necessary, but I want them alive. No disintegrations.
So their names and appearances are all G-level, but not their back stories. From Canon article on Wookieepedia :
G, T, C and S together form the overall Star Wars continuity. Each ascending level typically overrides the lower ones; for example, Boba Fett's back story was radically altered with the release of Star Wars Episode II: Attack of the Clones, forcing the retcon of older source material to fall in line with the new G-canon back story. However, this is not always absolute, and the resolution of all contradictions is handled on a case-by-case basis.
