I think after 8 weeks, pups will start getting pretty rough and need some really strict management when they are together. The pups that are dominant will possibly give the ones that are not as outgoing less confidence....and there may be some ganging up too. Or resource guarding if they are fed together.
Not sure if mom would be into managing or dealing with all those teeth coming at her from every which way!
Imprinting stage is huge during that time, I'd rather see the pups worked with individually, I don't think there is much of a benefit in leaving pups together longer than 8 or 9 weeks.