You don't have to sacrifice the Shadowborn Apostle whose ability you activated. It can be six other Shadowborn Apostles.

Shadowborn Apostle's first ability lets you ignore only the "four-of" rule. It doesn't let you ignore format legality.

You sacrifice the six creatures when you activate the ability. You can't respond to that ability by activating the ability of one of the other Shadowborn Apostles you just sacrificed.