That's not a bad video, tho rather shallow on info, probably the only viable chicken info I've ever gotten from TSC.....
.....now if they'd only stop selling those stupid tiny 'cardboard' coops.....haha!
Bringing adults into a flock of chicks is probably much the same except the adults could kick the chicks butts due to sheer size.
You'll need a 'look but no touch' area for the adults for several weeks, that will give your existing birds some more time to grow more and become a 'unit..
Lots of space, with places for the harassed to refuge, multiple feed/water stations, etc.
You'll have to watch carefully for injurious trouble when you finally let the adults mingle with the younger birds.