First, make sure the chicks look healthy. No runny noses, ruffled feathers, messy bottoms.
Ideally, you should isolate any new flock member from your established flock for 2 to 4 weeks....out of sight, out of draft, no shared feeders or waterers.
However, not everyone does that (and some to great regret) as viral, bacterial, and parasite diseases can spread, such as coccidiosis and Marek's.
If possible health issues are not a concern to you, and it is less risk at this point than at say, 4 months of age, or older, then it is a matter of integration of the birds themselves.
At 9 weeks of age, your little flock has only just begun to establish a pecking order, thus it is not entirely settled yet. Chicks at this age level actually integrate pretty well when placed with chicks of similar ages.
Just watch for any bullying and make sure there are ample feeders and waterers.
The greatest concern is health safety.
LofMc