1. At about 7 weeks you can start giving them a couple scraps. Make sure you give them grit so they can digest it if you haven't.
2. Feed it sparingly, think of it like candy. If you give them too much they can easily get overweight.
3. I would wait until they're a little bit older, at least around 4 weeks when they have most of their their adult feathers. If you could separate the run with chicken wire and let them see each other during the day for a couple days, that would be good. If you can't, maybe bring them out around the run for the same effect. When they're used to that, one night you can put the younger pullets on the roost with the older pullets. Watch them carefully the next day for any signs of bullying. If you are experiencing problems with bullying, one thing that has worked well in my experience was putting pennies and rocks put in a tin can and covered with duct tape. Just sit outside for a couple minutes a day and whenever a chicken starts to chase a younger pullet, shake it immediately. They dislike the noise.
4. After a couple of times giving them treats, you can put some of their favorites in the coop where they can see from the outside. If they still refuse to go in, then you might have to pick them up one at a time and place them in the coop, but they'll figure out that it is their home soon enough.
Good Luck!