General space minimum here is 4sqft per bird in coop.
1.8sqm is 19.3sqft, with 5 birds is 3.8sqft per bird, with 4 birds is 4.85sqft per bird.
I'd start integrating the 4wo chicks now, the older birds may be less threatened by smaller chicks. Maybe put the brooder box/run right in the main run...