The unanswerable question!
I would give the new kitty some extra attention - separate from the others - and do the same with the others. At the same time, I would look for another good home for the new kitty just in case you are not comfortable with the outcome.
You know, I am the person who took on Gabe, Pam's daughter's Ragdoll cat that was a bit of a monster cat. And as no one would have predicted, he adjusted to my cats very quickly and nicely, although Miley, my female cat is FINALLY, after 5 months, becoming comfortable with Gabe. Their relationships (all three) with each other have changed and finally settled into a pecking order. You have many cats and know all about it. If you feel the new kitty would be happier with people in a new home, then by all means, I would find it a new home. Word of mouth is a good way to find a new home. I would not take it back to the humane society unless all hell was breaking loose there at the homestead!
I would wait it out just to make sure your observations are accurate and look for a new home at the same time.
Keep us posted.
It is true that some cats need an environment change and it is nothing you can predict or should feel guility about. Cats are cats. They come in all shapes, forms and PERSONAILITIES!
I have three, with three totally different personalities.
SAS