Also...out of curiosity (I have no idea how 360 discs work XD), why would adding the male idols be at the cost of the 4 idols that can't be produced? It's not like they have more than one song (or, for that matter, more than one stage), and they probably don't have an insane amount of interactions (if anything, it'd probably be like how you interacted with the 961 idols in SP; when you decided to get sidetracked on your way to/from the office).
As far as a 360 disc goes, it wouldn't really, unless you're really pressed for space.
However, when you think of it from a company perspective, you've only got so much resources to use. The time, effort, and money you sink into these new guys will inevitably take resources away from anything else, like putting Ryuuguu idols in.
961 idols... were I think a little necessary if they were to bring the conflict straight to your face. I mean, Kuroi's a shadow character. Even if he is plain evil, he's kind of... a floating state of evilness in the world. He can't really do anything directly against your idols, since he's not one himself, so he has to get his own. When I started realizing this... I also realized that making these idols would require some time and effort on their part, and the only reason I doubted this was that I wasn't exactly sure if they'd actually sink those resources.
Though, I personally think the cost of adding in the 961 guys would be much, much less than the cost of making communications and event dialog that adding in the Ryuuguu idols would need. My stance on Ryuuguu, though, is unchanged, though: they simply didn't have the resources for all 13. They probably didn't just cut those four, either: it was an informed decision they made based on several factors, then they justified it all plot-like later. No one would make a decision that would endanger 25% of their core (emphasis on that word) fanbase if they could help it, and if forced to it, they wouldn't make it lightly.
... Although I can't imagine how it's like over at NBGI now. And I can't imagine what it was like before.