Unless canon explicitly says otherwise, I am going to assume that everything in core and HoH is available.
Other supplements should be approved by the troupe on a case by case basis. The main point is that I want to avoid being derailed by a load of rules not everyone has read, but if we agree on a few books and chapters to read, it is less of a problem.
Note that we need to create characters who fit together. I am planning to run 3±1 introductory stories, so that each magus and companion appears in one of these stories, filling them up with grogs. There should be some joint backstories between these groups as well, but not everyone has to know everybody else.
I want to hear the brain storming of character concepts before I decide exactly what those stories ought to be. After the intro stories, everything is going to be a lot clearer, and one can start talking about covenants and whatever else.