This is just a quick take on what Unity users seem to believe they need to understand inside and out: Quaternions
The Unity documentation for the Quaternion class is all you need to reference and use as a black box to the magic numbers of Quaternions.
If you understand Euler Angles, you can get along well enough to work with Quaternions. You don’t need to understand the inner workings of Quaternions. Unless you are really interested in them, spend that extra time making a fun game.
I used the Quaternion class with Unity’s Physics for Absolute Territory and A-Spec First Assault. It was more difficult for using use rotational forces rather than the built-in rotation methods made available.
As a side note: You use the Rigidbody Inertia Tensor for mass with rotations, and Mass for translation if you want to apply forces to simulate a Reaction Control System.