I know Project Fi recently rolled out and currently requires a Nexus 5x or Nexus 6p. At the time I'm writing this, no other phones currently support Project Fi, but are there any others in the pipeline that will work, which will be released (or have been released later)?
Project Fi works with the Nexus 6, Nexus 5X and the Nexus 6P.
Here is what the official site (FAQ) say:
When will Project Fi support other smartphones?
The Nexus 6P, Nexus 5X and Nexus 6 are the first smartphones that supports Project Fi's network of networks. During Project Fi's Early Access Program we plan to only support these devices.