Yesterday Project Fi became Google Fi and they now allow many different phones, although not all will support the ability to switch between networks:
https://fi.google.com/compatibility
Google Blog:
https://www.blog.google/products/project-fi/bringing-google-fi-more-people-android-and-ios/
https://fi.google.com/compatibility
Google Blog:
https://www.blog.google/products/project-fi/bringing-google-fi-more-people-android-and-ios/