No worries mate!
Umm, yeah that could be a way of "tricking" the ROM into believing you're playing just one instrument. Have you tried to use Anvil Studio for changing instruments? That's what I use, and that's what I think is the most easiest way of changing instruments and changing MIDI Events (like program changes, to change instruments.).
The 8-bit sounds would definitely be playing because they instrument, well, simply doesn't exist within that voicegroup! If you have more than however many DS tracks you can have in Ruby, the song will just either crackle or some parts of the song will not be played.
So I'd recommend you have a go at Anvil (personally, I don't know which is easier because I don't know how to use FL Studio, so it may be easier for you), and see how you go with that. And yeah, AFAIK Overdriven Guitar and Choirs aren't in Ruby...but I don't hack Ruby at all, basically, so yeah.
Just have a go, and see what you come up with!
And if you need to ask anything else, go right ahead and I'll reply whenever I'm free. :) Cheers