I agree.There's a difference when you get them all together than already have one or the other and are bringing in new ones to integrate. you can not just put them all together and think they will live happily ever after.
I added week olds to several month olds. All was fine for months. My experience may not be the same but I like to warn of potential problems too so that everyone is aware that things are not always good. I always recommend having a back up plan. Regardless of what you are doing. You don't want your animals to suffer because you weren't prepared or thought everything would work out.
I may do geese again. But they will be housed seperately and likely have their own run. They are so large, if you get a goose or gander that is not happy with another bird, the other bird will likely lose.