Hi Karen,
They usually start to grow feathers after a few weeks and will be able to go out fairly soon after they have a reasonable amount on their wings and chest- especially if they are in a small group as they can cuddle up for warmth if necessary. Mine go out at about 5 weeks but only at this time of year when it is mild. They get shut in a draght proof house at night and I only give them a bath on a sunny day when they can dry off without getting a chill.
You shouldn't put them in with your others because the first thing the drake will do is try to bonk them :roll: ... this is typical behaviour and he can cause damage when they are young. I will put all of my ducks together over winter. I guess my youngest ducks will be 20 weeks old by then. They all get taken out of the field onto a paved run so they don't turn it into a mud bath!
It is a pain if you have only one run but maybe you could divide the run for a while?
Tim