Good info from Deb.
From A breeders point of view. It is easier to house separate sexes as it takes a while to break a bond if Birds are to be set up with different partners to keep a healthy gene pool. Budgies can be stubborn and fall in love with the wrong potential mate just like people do.The age of the birds plays a big role. Babies need to be mixed as well as mature adults Who are retired from the breeding program, While vigorous youngsters need guidance. This follows through in how many birds are correct to house together. Breeding birds need more room than young chicks and elder adults who do not appreciate having their feathers pulled on by the younger set. So things can evolve over time and adjustments made to reflect the character and sexual needs as the flock evolves.
Best wishes, Jo Ann