Yep, I'd leave them with the broody for a bit longer - they'll need the heat for a bit longer as well, won't they? Although I guess it's already getting a bit squished until her by now. My ducklings went in with the adults at about 5 weeks, I think. By then they were big enough to look after themselves and it was no problem. At the moment I have six drakes and eleven ducks in together (along with the chickens and geese) and all has been well so far but I am going to sell off a couple of pairs/trios once I've managed to pick the ones I want to keep for breeding (twelve of them are only nine weeks old so I want to see the older plumage first).
H