can I use a "combat action" when the unit isn't in a combat.
Example: "Wildrider Squadron" is alone at planet 2. Can I move it to planet 1 during any action window in combat phase? If yes, I assume I could even move the unit to any empty (adjacent) planet?
Yes and yes. Unless the card states otherwise, a combat action only requires an action window in the combat phase. No other restrictions apply.