"that scale would be called G Mixolydian I believe.. but it's still in the key of C major."
that mode is g mixolydian, but it´s not necessarily in the key of c major. scales and modes don´t really have keys. songs/compositions have keys. although g mixolydian has the same notes as c ionian, the song could be in a minor. keys always indicate the harmonic center of whole songs or sections of it. for example "tears in heaven" (couldn´t come up with anything better), "all along the watchtower" and "so what" are all made of the same scale but are in the keys of c major, a minor and d minor. if you play a common blues in c (c - f -c -g- f- c)the melody could use any scale or mode; c ionian, c mixolydian (g major/ionian), c dorian (b flat ionian), c aeolian ( e flat ionian), c minor/major pentatonic...
don´t mix this up. you can play a song in a certain key but use different scales and modes to make up your melodies. the different modes have different flavours. it all dependes on the sound you want to create.
a good exercise to learn the sounds of the modes is to take a simple melody (e.g. the sesame street song) and instead of playing it in ionian, play it in dorian, phrygian, mixolydian... nice little game nerds like me used to play in mucic school.