Ok here goes:
Mysidia song is 1017 Bytes long.
Cyan's theme is 1039 Bytes long, so this is OK. - This is done already.
Shadow's theme is 859 Bytes long, no good. - Looking to see how to do this.
Update:
I've tried replacing shadow's song but one of the instruments messes up right at the start, and then is never played again. It's because of the reduced rests between pieces of code. It may be fixable, but I don't know how atm.
Another way this can be done is inserting mysidia's theme into cyan's theme, and then replace all instances in the code in which shadow's theme is called by cyan's as well. If you go to the event dump, all pieces of code I could find in which shadow's theme is played in the game is:
Code:
CA/EC45: F0 07 Play song 7 (Shadow), (high bit clear), full volume
CB/0B78: F0 07 Play song 7 (Shadow), (high bit clear), full volume
CB/CCDE: F0 07 Play song 7 (Shadow), (high bit clear), full volume
CB/CD27: F0 07 Play song 7 (Shadow), (high bit set), full volume
CB/CD62: F0 07 Play song 7 (Shadow), (high bit clear), full volume
CB/ECAC: F0 07 Play song 7 (Shadow), (high bit clear), full volume
CC/6F63: F0 07 Play song 7 (Shadow), (high bit clear), full volume
if you want to go with this let me know, I think this way shouldn't be that troublesome