Hi, I've used grobags with my four. When I had the first 2 the recommended size to start was, I think, 10lbs. For the last 2, the 0-6 month one's design has changed and you're right - it's 8 lb 8oz minimum weight.
Despite having the bags before they were all born, all of mine spent the first 4 weeks at least swaddled - they've all loved being swaddled! Because newborns are just sorting out their reflexes, they wave their arms a lot when sleeping and wake themselves up (v.v.v. bad for your sleep!) by hitting themsleves, but swaddling helps contain that.
I found grobags brilliant once the babies were a few weeks old - they've travelled with us all over the world and slept well everywhere, smelling of home, I suppose?
I've always had 2 of every size - expensive initially but pretty good over 8 years! They wear well, wash easily, and the oldest ones in our house have been used daily for eight years now. You don't need any other blankets at all with them as you layer up the baby according to temperature with an extra vest/not.
My eight year old has in fact gone to bed in his size 6-10 year one tonight, such is his love for grobags!!