I linked my integration article on page 1. However, I do not free range - do you have a run available that the chicks can use, so they can see the adults first before coming into contact with them?
Only issue with the "add them at night" is your flock will notice the new birds the next morning and there might be quite a bit of tussling when they do.
If that's how he's been doing it and he's had good results, I'd at least copy his set up (clutter in run?), timing (like opening pop door at...
You can start introducing them now but at this point they should be kept fully separated and I would continue to have them fully separate (but in line of sight of each other when they're out) until the hens don't show interest in them.
You can integrate early IF you have the space and set up...