That it was not in Crisis Core is like an extreme betrayal to Sephiroth. Make Chapter 8 the Crisis Core, skip the part where Zack finds Sephiroth and make it a DMW instead, which basically means the beginning of the Crisis Core is not even shown obligatory and then not even play Those Chosen by the Planet. There is a reason it is called "Crisis Core", it leads to and features the Nibelheim Incident.