I’ve been doing this early integration thing for forever, it seems. When I first did Mama Heating Pad in the Brooder  and posted about it, most people were shocked and warned of dire consequences.....and I got some pretty nasty PMs about my cruelty, stupidity, and killing my chicks because I was trying to show off. Then I moved my chicks out with the adults.....gasp, shudder!!!  More nasty PMs.  If I heard, “The experts say...” one more time I was going to scream!  And I still hear it, but I just ignore it.
When I have chicks out in the run in their wire pen, I hang the adult and the chick feeders and waterers right next to each other.  They learn to eat and drink together peacefully with the wire separating them.  I use 
@azygous portal doors system.  I start mingling chicks with the flock at 2 weeks, they’re out with the adults all day long at 3 weeks, and fully integrated with the flock by 4 weeks - and that’s also when the brooder is removed completely.
The real experts are good broody hens.  They get it right 99% of the time if not interfered with. So everything I do is designed to mimic her as closely as possible, including introducing chicks to the rest of the flock. The chicks thrive, and I’m not stressed out and fussing all the time.  I hatch ‘em or order them, show them where “Mom” is, move them out and then feed and water them. I don’t depend on anything but common sense, and once in awhile a super helpful idea like the portal doors I stole from 
@azygous when she started doing it this way.  The heating pad for heat idea came from Patrice Lopatin and fine-tuning came from my much-loved mentor, 
@Beekissed. I’ve never looked back, and I’ve never had an adult even injure a chick, much less kill one.  That’s 8 batches of chicks. Same method every batch, every time.
If a two pound hen can do it without experts, books, websites, or any other artificial means, why do we do it so differently and think we’re doing it better?  A lot depends on our own personal comfort zones, I get that. This is mine.