That's usually the case once a silverback passes - however it can at times present issues amongst the girls who tend to fight and bicker more often without the presence of a silverback. In a normal functioning troop, the silverback is the one who will maintain order within the group.
As @Pertinax said, Kibo's still young so they'll likely have to wait another few years until they're confident he won't be at risk during introductions.
Also, where a female takes over the position, it doesn't mean there isn't any stress in the group either. A female doesn't hold this position by choice and it's more as an interim until a new male arrives. She, as well as the others,may herself be stressed/unhappy by the absence of a male. But with a youngster in the group there is little they can do. I would say four years old might be the lowest threshold for bringing in a strange male though some places wait a lot longer. What they have to weigh up is the risk to the youngster against the continued well-being of the male-less group.