iOS App Preview videos - are all 3 iPhone resolutions required
In the end I only uploaded 3 videos - iPhone 5 resolution, iPhone 6 resolution and iPad resolution. When I visited the app on the App Store on an iPhone 6 PLUS - the iPhone 6 video was shown.

So it looks like larger iPhones will fallback to smaller videos if the larger preview video is not present. Presumably the iPhone 6 will fallback to the iPhone 5 video as well.