re. "their instrument banks were effectively hard-coded", the Creative Music System was a PSG-like chip with only square waves and no sample ROM at all, the AdLib was a FM synthesizer with no ROM banks either (though some MIDI banks which sounded nothing like sample-based MIDI players like the SC-55), and the GUS had a mix of ROM and RAM (and I don't think it targeted General MIDI either). I don't see any reason why any of these chips would be incompatible with trackers, and I do know the AdLib had multiple trackers historically and today (Reality Adlib Tracker, Adlib Tracker II, ScreamTracker 3, and more recently OpenMPT), though I think "feeding MIDIs through per-game custom soundbanks" was more common in video games, and AFAIK the GUS had more MIDI-based composing tools than trackers. Note that I don't know much about the CMS and GUS, and I have more experience with how Adlib music is composed/played today than in the 1990s.
My recollection is that while some PC sound cards of the era technically supported arbitrary FM and/or wavetable synthesis, it was difficult or impossible to get any custom sounds out of the MIDI side of them. I remember there typically being the General MIDI sound set, and maybe some custom presets on the higher-end cards. The big exception I ran into was the Yamaha DB50XG, which had an unbelievable (for the era/price) synthesis engine, but I had to buy third-party software to customize the instruments. I heard stories that implied vaguely similar things about the Gravis Ultrasound, but never owned one myself.
I remember also that ScreamTracker was able to use Yamaha OPL series FM synthesizers, and the author detested MIDI because it did not give enough control over instrument acoustics.
re. "their instrument banks were effectively hard-coded", the Creative Music System was a PSG-like chip with only square waves and no sample ROM at all, the AdLib was a FM synthesizer with no ROM banks either (though some MIDI banks which sounded nothing like sample-based MIDI players like the SC-55), and the GUS had a mix of ROM and RAM (and I don't think it targeted General MIDI either). I don't see any reason why any of these chips would be incompatible with trackers, and I do know the AdLib had multiple trackers historically and today (Reality Adlib Tracker, Adlib Tracker II, ScreamTracker 3, and more recently OpenMPT), though I think "feeding MIDIs through per-game custom soundbanks" was more common in video games, and AFAIK the GUS had more MIDI-based composing tools than trackers. Note that I don't know much about the CMS and GUS, and I have more experience with how Adlib music is composed/played today than in the 1990s.