Revision. I know the longer usernames would be cut off, but you could tap the profile picture to bring up a more information window which would display the full username along with all other user information. Windows Phone cuts off names of people you add to the homescreen, so Microsoft obviously don't see it as that much of a problem. You have the first part of the usernames and the profile picture, so you should be able to identify the user with no hesitation anyway.
