It reads quite clear to me. after reading this thread and the text again. The "only" in questions relates to the conditions under which the Krox can be attacked. "...can only target Kroxigor within a mixed unit if..." says that if the condition (in b2b contact with a Krox or b2b contact with a skink in b2b with a Krox) only then can one target a Krox. It does not say one must target a Krox if the conditions are met. Furhtermore, earlier in discussing the Spawn-Kin rule GW uses the word must wrt to putting Krox in the second rank if 5 or more skinks are in the first rank.