?????
Dude, what are you even getting at?* It's all made-up stuff that changes from edition to edition, book to book, and you can do whatever you want in your home campaign.
I just use the original source- do what you want, and argue with someone else who doesn't know any better.
*I literally said the following that you are arguing with, when you said that they didn't co-exist:
"If you place them as denizens of Limbo, like the Fiend Folio did (saying that co-exist with the slaadi), then they would have to be CN like the plane, right? That's how 1e cosmology works."
But sure, if you assume I meant that I wasn't talking about the Fiend Folio, or 1e, whatever. I mean, why should I even bother stating that I am using the Fiend Folio, and discussing the first edition if you're going to ignore it?