I "add" new birds every three weeks - however many hatched of my latest incubation, meaning typically six to ten. Adding in larger groups appears beneficial, as the group tends to operate as a flock within a flock for mutual protection, but I'm integrating with birds 3, 6, 12, ... ...60 weeks older than they, so there's quite the range of sizes and relative maturity.
Two keys - one, a lengthy "see and be seen" process, where the existing flock can get used to the presence of the new additions, yet be separated by a see thru barrier like a cage, netting, chicken wire etc while they adapt to one another's presence.
Two, abundance is a social lubricant. Having the two flocks eating next to one another (separated still by wire, net, etc), drinking, playing, etc also helps reduce stress and improve acceptance - the birds don't feel like they are competing for resources.
Third, and not a key, just a matter of acceptance - "pecking order" is a thing. There WILL be pecking involved. As long as its not resulting in open injuries, some pin pricks on the comb from another's beak or the like are perfectly normal behaviors, as is chasing and jumping. Don't panic. IF, otoh, one group starts to gang up on an individual and attack the hind quarters mercilessly, step in and separate them again. That's a particularly vulnerable location, and can go south quickly.
Damage from chasing, jumping, defensive behaviors can be minimized, in part, by ensuring there are lots of obstacles in the run when the barriers are removed, so the smaller birds have places to hide, exit routes, and ways to break line of sight. While birds do have some sense of "object permanence" in their dinosaur brains, they remain quite visually focused. Breaking line of sight rapidly defuses most situations, I've found.
I have no advice re sleeping behaviors and small coops - I know some will pick birds up and place them together at night - I don't. But I have multiple coops and multiple large runs, so I don't experience the space pressures other backyard chicken owners do.
Hope it helps!