The fast answer is YES, all Harmony editions are compatible together.
We moved all of our animation products to the Harmony family to make all of our products compatible together. With the three editions of Harmony, we wanted to make sure every artist using our software would be able to use and collaborate with any other artists using any other software from us.
There is one thing to keep in mind though, the higher editions have functionalities that do not exists in the lower editions. However, projects are compatible and will open in the lower editions. Anything done with a functionality that is not available will be visible but not editable
I hope that answers the question.