How much space do you have...in coop and run in feet by feet?
Do you free range at all?
How old are all the birds?
Do you you have a separate but adjacent enclosure so the birds can see each other but not touch for a couple-few weeks?
It's not so much about size/age as it is about territoriality....the existing birds are
not going to be happy to have 'strange' birds invade their space.
Lots of space, multiple feed/water stations, places to hide 'out of line of sight'(but not dead end traps) and up and away from aggressors all help the integration go smoother.
Consider biological/medical quarantine:
BYC Medical Quarantine Article
Poultry Biosecurity
BYC 'medical quarantine' search
Read up on integration..... BYC advanced search>titles only>
integration
This is good place to start reading:
https://www.backyardchickens.com/a/adding-to-your-flock
