Yep, and mostly dealing in puns (which are awesome, I love puns).
Agreed. Leitmotif is a song that evokes the character (or player in this case). This is a nice podcast (and series if you want to learn about classical music) about John Williams' use in Star Wars: https://classicalclassroomshow.com/classical-classroom-episode-4-brett-mitchell-teaches-leitmotif-in-john-williams-star-wars-score/
Yakety Sax makes you think about running around, so stick that on D that runs around in his own zone a lot. Holst's Mars, Bringer of War would be my pick for John Scott, since the song just sounds big and ponderous. Although, if you wanted to stick with the Holst thing, Jupiter would fit too since it's big but also jovial.
I'm struggling to not use classical/orchestral music, since I'm not sure you can really do leitmotif with a song that has lyrics and popular music usually does. Plus, Mars is a banger.