Any old time. Just do it smart. If they can see each other and hear each other from their current coops and have done so for some time, just combine them at night and in the morning they will sort it out.
If they haven't, maybe you can arrange that they can be combined without getting to touch each other for about a week and the integration should go a bit more smoothly. That's how the coop and pen crowd seem to work it out and by all reports this is the best way they recommend.
I free range, so these issues don't happen for me...I just throw everyone together and they slowly become a single unit as they forage, eat and move through the day together.
There will always be pecking order that has to happen, so just give them plenty of space and let them settle things when you finally put them together.
I am new to this as well....
I just successfully added two 7 (1 silkie and 1 speckled sussex) week olds with my 13 week olds(2 barred rock and 1 buff).
I put a wire dog crate next to the run during the day for about a week.
While I was with them I let them roam around the yard. There was some pecking and chasing but them pretty much kept their distance.
Next I physically put the crate inside the coop. The big girls climbed on it and were curious but everyone slept pretty soundly.
During the day I let them all in the run together. I made sure to be close by for the 1st few days. If i couldn't supervise them off to the crate they went.
The big girls would stand their ground eat 1st then settle in. The little ones knew their place and patiently waited.
If they did get took close they would get a quick peck correction and retreat.
I also Hung bagels and corn from string just out of reach so there was a lot of distractions. plus it's so much fun to watch them jump and "play".
This went on for about a week and a half until one night I forgot to close them up for the night. When I went out, the little ones were all cozied up and the big ones were in their usual
position on the roost. Once that happened I removed the crate and they are all living comfortably.
It took time but it appears to be successful.
I have two silkie chicks that are two weeks old. I will definitely use the same plan when their time comes.