It'd be an interesting touch if that thing popped up as an end credits teaser or something in Sonic Adventure. The golden glow surrounding Super Sonic is a nice touch, you managed to make it noticable without going overboard or obscuring his features, which isn't easy!
As for Generations, I have my own theories on that design...
Well, two things to keep in mind: not only did the creature in Generations look different, but it fell to normal Sonic, without him having to go super.
The other bosses in "Generations" didn't need Chaos Emeralds for power, but they didn't need Super Sonic to beat them the first time around, either. We saw that Sonic was collecting the Chaos Emeralds, and I get the impression there were never more than seven total in that whole screwed-up timeline.
Most likely, Perfect Chaos was forced into that form by the Time Eater's energies, but was forced to drain power from another source (Probably the Time Eater itself) in order to sustain its Perfect form. This changed its appearance, and also weakened it, which is why regular Sonic was able to beat it.
(I actually like the new Perfect Chaos design, just not as much as the original. But that's just me...)
What you mean is that the SG design was the one they meant to use for Adventure (evidenced in the Lost World level at the end, where the mural clearly depicts that design). Am I right in interpreting it that way?