Something else that's interesting: I just checked X6's manual to see if there was anything additional about Signas and noted that, at this point there are still Units in the Hunters:
MEGA MAN X Mega Man X leads the 17th Reploid Hunter Unit. He's working on the Earth recovery operation with other Hunters when he receives Alia's SOS.
of course, the Signas blurb tells us veeeeery little here:
SIGNAS A young Hunter general, Signas has gained the corps' trust and loyalty with his cool judgement and level-headed leadership.
...I THINK we may be able to put this shit to rest. Unit 0 still exists as of X7, and X is only referred to as the former leader of the 17th because he quit fighting.
It's only in X8 and CM that X and Zero are referred to as 'former' unit leaders; it's that recent of a development, storywise.
So what the hell happened between X7 and X8 to disband the Units?