Integrating chicks is much easier than integrating older chickens, even the "teen-aged" ones. And, generally, chicks don't present the health issues of older birds. They haven't had time to develop immunities to to something at their "home" locations and bring in that something your first batch of chicks haven't yet encountered.
Although it IS one reason why young chicks not hatched by your own hens should not be integrated with older birds until they are past the first 12 - 16 weeks. They're pretty vulnerable up until then.
I'd go for it. I HAVE added chicks at 1, 2 and even 3 week age differences. (Right now I have two separate brooders because the older chicks are standard breeds and the newest chicks are bantams and still less than 5 days old, so they're still little, itsy bitsy things about the size of large cotton balls. The older chicks would bowl them over too easily right now.