One developer is more than enough for most iOS applications that doesn't cross into games and physics.
You also don't need an "Mobile UX, UI, iOS designer expert" either. Just get a good developer that can do both, instead of mixing science with snake oil.
In the end, one guy to be the product owner, develop and design will be leaner, and far more better than involving more people.
Less is more.
It's easier to replace one entity with another, than several parts.
Knowing the SDK and design elements beats a designer that only knows design.
And the API consumer would probably develop a better API than some guy who generally doesn't consume most of his API for a living. That being said, that assumes the front-end guy also knows back-end. Rare, but exists. points to self
I generally dislike "Mobile UX, UI, IOS Design expert" if they can't code their design. Either for Cocoa or (S)CSS CS/JS depending on platform.
Also I think if you hire an expert in field x, there is a lot higher chance that their coding style and practice will be easier for another expert in field x to pick up. Ever seen a developers photoshop files :p