Where do I ask questions on information on commercial music?
Eg: Chord progressions, lyrics, instruments used etc?
Can I ask it in "Music: Practice & Theory" or is there another place?
The "commercial-ness" of the music doesn't have any bearing on whether or not a question is on topic here. If it's a question that a musician would be asking about the theory or practice of that music, the question will likely be on-topic. If the question is something a non-musician might ask--or what we might call a "music appreciation" question, then it is not on-topic here.
It depends a lot on exactly what you mean. As I read your question, I'm pretty certain these kind of questions will be off topic here, but you can ask at the Music Fans site.
The beta site will launch really soon, so commit to it! Have a look at the example questions there to see what that site scope will be.