A tip for anyone struggling with completing the "All Boss Rush" mode (All Hero, Dark, and Last bosses),
The last boss of Hero and Dark side (Sonic vs. Shadow, and Shadow vs. Sonic), has infinite rings you can collect.
So you could just keep running forward, lightdash and collect rings to build enough extra lives to clear the "All Bosses" boss rush, or as many as you think you'll need.
(Edit: Also, you can bring upgrades into the boss rush modes as well, such as the Protective Armor that Tails & Eggman can get.)
Other than that, I wanted to say great guides!