Your little ones and older ones were always with other emus and were used to other emus, which probably made it easier. Berrigan was already ten days old when Ciara hatched and at that point had never been with another emu. And as soon as I tried to put them together he started pecking her. Probably just an exploratory thing, but he's bigger than her and he was almost knocking her over when doing it.
I'm not going to split the goose up from him because he freaks out. He freaks out if I pick the goose up for too long to hold him and talk to him, and he can actually see the goose while this is going on. He still starts crying until I set the goose down. At earsplitting volume.
Berrigan is doing to Ciara what Ciara does to new birds she sees and isn't familiar with - pecks them. I showed Ciara a gosling this morning and the first thing she did is start pecking at it, which is exactly what Berrigan does to her when I try to put them together. So it's nothing the goose is teaching him; it just seems to be what they do to unfamiliar birds. He may even just be trying to 'explore' her and learn about her, but he's bigger and it could hurt her.
So unfortunately for now they can't safely be together, and will have to be introduced slowly. Which is fine, I don't mind doing it that way.