A couple of them are tricky. Lobo has some prereqs, then you can't seem to get rid of him once he's done the HG/SG/WW M.B. scenes. As for Parasite, I think that he's condition-based as well.
I did discover that, for Lobo at least, you can't send him on heists once you've met his prereqs until the hideout is upgraded to level 4, which unlocks the 'next page' button. And there's a 'bug' with the 'next' button in the Lexcorp Defenses menu where the 'next' button can't be seen but if you know where to click you can still 'page' to the second page, that Lobo's on.
Metallo is tied to to the prison event, as I understand. I did note that once you've freed him after returning from prison, he doesn't report to the hideout, he just returns to Lexcorp headquarters after he's sent off on capture missions and such. That would fit with his dialogue/story if I'm remembering correctly.
Other supervillain(esse)s are also tied to the Prison, Yacht and other events, and won't become available until those events are completed.
That being said, in my current playthroughs I've been avoding triggering the Prison event by not completing the associated heist events, but have still managed to hire more supervillain(esse)s than I need. Between Cheetah, Giganta, Livewire and Star Sapphire, plus Atomic Skull, Shade, Toyman, Bizzaro and Sinestro, well there's only four positions in Lexcorp headquarters to man, plus one heist maximum each night, so the extra SV characters are essentially just gravy, except where they tie into storylines (such as Poison Ivy).
Sinestro's spawn rate is a bit finicky, so I suggest working towards unlocking Lobo as soon as you can. If you complete the first Grogg heist but not the second one (which triggers the prison event), Parasite should show up at some point. Not sure what exactly triggers his availability still, but I'm guessing it's the first Grogg heist.
I suppose that the $150k level is a bit light until the Prison event happens, but Toyman still has you covered if you are employing an 'all girl' workforce in the four assignments at Lexcorp headquarters...
So the other SVs and such are mainly tied to various story elements, and Sinestro/Bizarro are the only Class III SVs ($200k) that you really need for capture purpose. And Parasite/Lobo once you've unlocked them.
But you can unlock pretty much all of the Heroines once you've upgraded the Glamour Slam and Meta Bordello currently. Until new ones are introduced in future updates I suppose.
As for the 'caged' Supervillainesses that you acquire via various story arcs (Cheshire, Crush, Harley #2, Galatea, Superwoman) as well as Audrey and Lois, well they currently can't be assigned to the Glamour Slam anyways, which is what it is. I'd love to put the two Harleys to work in the Glamour Slam, but the best we can do right now is take them there on leashes, which I'm not sure if that helps at all with the income stream (it probably should).
I still think that Lex has a cameraman hidden on that street where he takes gals for walks, and is posting the associated videos on the internet himself, which should probably get him an extra small revenue stream and increase interest in the Glamour Slam and Meta Bordello (by advertising the goods), but as far as I know the Street scenes are mainly just for the jollies and possibly unlocking more sexy scenes later.
Hope this helps!
The one downside of this would be thumbnails if you use those. The thumbnails, at least the ones on the forum end, seem to be tied to the post where you attach the image, and I haven't found a way to invoke them again. I think you found a workaround for this though, by creating your own larger thumbnails?