I have pushed up my pairs by 2 - 3 pairs over the 1 pair per sqm rule, but there are other considerations like mentioned above. For example my one aviary is 3x2x2m = 6sqm. I have 8 breeding pairs in this aviary. 3 pairs gouldians (use nest boxes), 3 pairs golden-breasts (use wicker baskets) and 2 pairs blue-breasted cordon bleu's (build their own nest in shrubbery). If you can find a good mix between what nest types the birds use, level of nesting site in the aviary (ie orange-cheeks build at ground level, while cordons build up high) and can keep everyone happy in terms of their food requirements, then you can push the pairs up a bit. In this aviary I mention, all the birds are currently brooding, par 1 pair of GB's.
Good luck with your decision and you should definitely look into getting some cages if you don't have space for another aviary. We get too attached to these little birds just to rehome them because of space issues
