Any issues with merging chicks that are 8 weeks apart in age?

[NickyPick]

I hatched a batch for a friend on January 19th and then ordered some Brahma's for myself to come on March 7th. My friend has decided to add two brahma's to his new flock. What do we need to be aware of and careful of in merging the two Brahma's with the older group? (mix of Orps, Wyandottes & EE's)

 

[Fred's Hens]

The older birds will treat the younger with distain and as intruders. Integration is sometimes difficult, but eventually, it happens. Introduce them by having a separation between them for a few days. They'll see each, hear each other, but be restrained. I usually introduce, up close and personal, outside while everyone is busy. I do the actual combining at night, just at "going to roost" time. There'll be some issues the next few days, but normally, things settle down.

Of course, this is after both sets of birds are out of the brooder.

 

[ChickenCanoe]

That's tough.
Those birds will be 7 weeks by then.
I'd say wait till the chicks are feathered but by then the olders will be 3/4 grown and would probably pick on the little ones.
You'll want to quarantine anyway so after a quarantine period, I'd put 2 of the older ones in the quarantine pen with the young ones till they get acquainted and then integrate them.
The little ones will have bigger friends by then.
Lots of space always helps.