When Cad is adjacent to Luke GH and gets shot at he can use Hostage Shield on Luke. Luke mave and Cad takes the damage. What if Cad is adjacent to an allied mouse droid and wants to use Hostage Shield on the allied mouse droid. Do I have to make the save roll and fail it or can I just bodyguard it? Main question, is hostage shield an option or is the save always rolled, every time?

It's always rolled every time. Dominate set the precedent that you can't choose to fail saves.

"Saves are not optional, and characters cannot voluntarily choose to fail them."

Rulebook, pg 24