It is often easier to introduce a neutered boar to a sow group, as he will not impinge on the sow hierarchy. The only problem comes right at the start if the top sow and the boar can't come to an agreement s to who comes top. Hence the recommendation to date. Generally, it is easier to add younger sows to a group, as they usually don't have the wherewithal to contest the leadership and are more accepting of fitting in at the bottom of the existing hierarchy.