Is it mandatory to upload screenshots for all screensizes when submitting to the app Store?
From what I understand if you only upload 5.5inch and iPad, they'll just reject you outright. Currently, it looks like they require at least 4 and 3.5. If you indicate it's optimized for iPhone 6, it also requires a 4.7.

It looks like if it indicates that it runs on iPad you need both an iPad an iPad Pro. 

Check out the requirements for yourself here: iTunes Connect Dev Guide

Remember, required/requirements means it's mandatory.