I've posted my thoughts about groups/folders to dA, as requested.

> And how much of a piece has to be Inkscape-created for it to qualify?

I personally don't feel it has to be 100%.  As long as it's usage is not
insignificant it will be accepted into favourites.  But because the Gallery
is for exceptional Inkscape work, ideally it would need to be predominately
Inkscape.  It's subjective.

That seems reasonable, and in-line with what I was thinking.

Although I haven't said as much on dA, perhaps a "Mixed Workflow" folder might be a good addition, for works which include a significant amount of Inkscape work, but also a significant amount of work from other applications. The heavily GIMPed  pieces, or the Synfig animations which use Inkscape as a starting point. I think it would be good to showcase some of these works, to show Inkscape as part of a wider ecosystem of applications, but separating them out from the 90% Inkscape works would help to avoid any confusion as to Inkscape's own capabilities.