The highest leadership is taken. For fear, it works a little strangely because its a mixed unit. To beat and outnumber an enemy unit by fear, the kroxigors must outnumber them, you cannot use the skinks US since they don't cause fear. On the other hand, since the krox cause fear the entire unit is automatically immune to fear so will never need to take fear tests.
The thing to remember here is that fear tests are only ever taking if it brings you in contact with a fear causing enemy. In the case of Skrox you are not directly in contact and some people would say, 'no fear test for me, yay!' but, in the rules of the book, the Krox behind a skink counts as being in contact with the enemy touching a skink in front of it, so a fear test must still be taken. Other than that, what Strewart said.