These things were never intended for Generations, SEGA always had planned for the eras in the game. So how can there possibly be more than one Unleashed boss? Unleashed was a great game. Still can't be more from it, it featured as a level and a boss already. Not that I know this for sure, but I agree with the assumption that Generations was built on Unleashed.
...Please go to my YouTube channel and watch me play Sonic music on the piano: http://www.youtube.com/user/cyanpiano
Themes like Metal Sonic from Sonic 4 Episode II and Starlight Carnival are there!