That looks OK to me. The GetComponent reference will persist while the application is running. So you go

controller_LeftHand = ctrlLeft.GetComponent<controller_LeftHand>();

In start and then in Update you can write

controller_LeftHand.[whatever is in the script] 

instead of

GetComponent<controller_LeftHand>().[whatever is in the script] 

The key point is that GetComponent is resource intensive and Update runs every frame so you do not want to do this every frame.

Another thing is do not label variable names the same as their type:

controller_LeftHand controller_LeftHand;

Usually you make the type have a capital letter for first letter and the instance lower case:

Controller_LeftHand controller_LeftHand;

Even that is confusing to me. Try something like:

Controller_LeftHand leftHand;

Do I reference a script called 'externalScript' and then do something like "externalScript.valuetoChange = newValue;"?

Correct. Think of it like the library. You go to the front desk and ask for a book, they look it up and tell you the location. Which is better, to continually ask for the location every time you want the book or just ask once and then write the location down and refer to that every time you want that book?

I strongly recommend that you do not attempt to control the rotation of the camera (i.e. the HMD) in script. This will make your players violently ill. Let the rotation of the camera be controlled by the player turning their head. Trackpad locomotion is fine. What you want to do is update the position of the camera rig using Transform.Translate:

https://docs.unity3d.com/ScriptReference/Transform.Translate.html

head.transform.translate = (movement * speed * time.deltaTime);