Dear T. …
Do experiment with this idea. Readers love the mind manipulation of thrillers; you could do some cool manipulating with one 1st person narrator among several 3rd person narrators. Maybe the 1st person POV would establish your protagonist as the most trusted narrator, when in fact readers should be trusting someone else entirely. Maybe timing the switch to 1st person could alter the tone strategically—perhaps allowing readers to pull back from high action for a quiet mull in someone’s head, or maybe increasing intensity as the 1st person narrator pummels readers with a more immediate sense of emotion. I do hope there’s an underlying logic for the single 1st person narrator. I don’t want readers feeling like the author is being clever for clever’s sake. Your concern about clarity and reader comfort suggests you’ll handle this smoothly, so I’m not worried readers will struggle.