That's an interesting issue you bring up.
I'm specifically referring to the art "style." Something around 20k polys would be low-poly and then anything higher is high-poly. This would regard one single object, so a human model with that 35k would be high-poly. The scene total would still be low-poly even if it totaled 50k but each object was 10k.
I think this could be added to the artist options when uploading, to say whether it is high or low poly.
Though community tags could also be introduced to solve this issue.
For the quads, I was hoping that it would be possible for Sketchfab to de-triangulate models. Is it possible?
Just a generic automated quadrify.