Will App Preview video for iPad be shown for iPhone users?
No, the iPad 'App Preview' video will only be visible for users who are viewing your app page from an iPad. 

For iPhone users you would ideally upload three video at the following resolutions: 640 x 1136 (iPhone 5), 750 x 1334 (iPhone 6) or 1080 x 1920 (iPhone 6 plus). If however you decide to only upload one video (say 750 x 1334) larger iPhones will fallback to smaller videos if the larger preview video is not present.

N.B. These resolutions are for portrait mode.