Great Reach: Kroxigors in mixed units can reach over a single row of Skinks to join in a close combat as follows. If a Kroxigor is in base contact with a Skink of its own unit which in turn is in base contact with an enemy, then the Kroxigor too is counted as being in base contact with that enemy. This means that it may attack and be attacked by that enemy model. "This means that it may attack and be attacked by that enemy model." So that means that the skinks don't provide any protection for the Kroxigors because the enemy can just target the Kroxigors instead of the Skinks. So what's the point of putting Skinks in your unit with them? It's kind of just a waste of points imo.
Actually its pretty much the krox that protect the skinks from shooting... There are several advantages. Firstly, skinks provide some static combat res. They get the numbers up and maybe have a rank or two. Of course, you need to pick your targets carefully or the amount that die will negate the static bonus. Secondly (and probably most important) when the krox are in a skink unit, the whole thing contributes to one of your minimum core choices required rather than taking up one of your special slots. Thirdly, think of it not as an upgrade for the kroxigors (giving them protection etc) but an upgrade for the skink unit, you need to use the unit like it is a unit of skinks not like it is a unit of kroxigor. This means that since the kroxigor cause fear you have effectively granted your skink unit immunity to fear, which is a big boost given the number of fear causing units and even armies getting around now. Think about what you want the unit to do. If you are looking for a hard hitting hammer unit, go for the krox on their own. If you want more of a support unit, then take skinks with them.
One of may armies has 5 units of skink/kroxigor combos. Two kroxigors and 16 skinks, a banner and musician. It is suprising how many times these guys stay in combat. They have even worked well as a flank unit against really slow armies, etc. Abot as many points as a typical saurus block? When combined with multiple EotG's running in between them, they become even more dangerous. The Engines can do some burning alignment to weaken the units you would like to charge with the krox/skink units before you charge. You can also use buff spells, movement spells, and other enhancements to the Krox/skink units. One of my favorites it to make the enemy WS 1 before the combat begins. Having the Kroxigors gives the immunity to fear as has been mentioned, and it also increases the leadership to 7. With the musician, you make that even better for a rally. Last but not least, they can still shoot and even stand and shoot. When there are 6 of them across, not so bad.