the musical modes (note title) in their traditional order, though I confess I don't understand all the wordplay here. Readers in tune with their inner punster will note the harmony between the theme and the wording of the clues, which is key to the puzzle.
1. Early scientists see major progress.
IONIAN; something to do with the ancient Greeks? (And the Ionian mode is more or less the same thing as our major scale.)
2. Gray scale picture subject?
DORIAN (reference to Oscar Wilde)
3. Midas tunes up this hangman cheat.
PHRYGIAN (Midas was king of Phrygia; perhaps the "hangman cheat" bit is a reference to the "Gordian knot", though that doesn't seem to involve anything hanging?)
4. Endlessly in Whiterun; Croesus is the key.
LYDIAN; Croesus was king of Lydia. In the game Skyrim, Whiterun is home to Lydia, a prominent follower for the player.
5. Shuffle mode of 4?
MIXOLYDIAN (mix, ouch)
6. With a minor change, sounds like garlic mayo.
AEOLIAN (aioli, ouch squared; and the Aeolian mode is more or less the same thing as the natural minor scale)
7. Made successive notes in his ancient Opus.
LOCRIAN but I confess I don't yet understand why.
The missing tag is obviously
the OP's username is most appropriate.