Frenzy doesn't carry to the unit.
See the main rulebook page 74-75. On 74, under Shooting, it says if a character model including mount are US 5 or more then they can be picked out as a target for shooting with no penalty. On page 75 under "look out sir", it says a character with US 5 or more cannot benefit from LoS. Large target has nothing to do with it.
Slann cannot be hit because they are considered "man size", are less than US 5, and sit in the second rank. Kroxigors in fact CAN be hit by shooting, if you look under the rules for kroxigors in the LM army book, you will see that it says missile hits are randomized beteen the skinks and the krox.