I'm preemptively asking if music made by Joel is acceptable to use once development starts for a few reasons. I'm worried they would be flagged on Twitch, or disallowed on Itch.io, or that it would be too lazy for the jam, or Joel wouldn't want it.
Any answer is appreciated.

As-long as you credit it, it should be fine. I doubt Skeleton Metal is set up to get flagged on Twitch, but if I find out otherwise I'll let you know.