Based on the wording confusion that we see with the rule. If the word 'only' was not in the sentence, then it would be clear that enemy units may attack either skinks or kroxs when the conditions are met. Since the work 'only' is included in the rule, I would agree with original poster saying models MUST target the kroxigor in combat when the conditions are met. This is definitely Rules As Written.
That RAW isn't broken since the skink to krox ratio is still 8:1 in a unit, so there are very few wounds of kroxigors in the unit that the enemy must kill before skinks are targeted. Plus kroxigor attack last in combat (and hit hard) thus the enemy would probably WANT to attack the kroxigor so they have several fewer attacks coming their way. However, this would give characters an unprecedented amount of protection in close combat (not that skink heros are that scary in close combat). Skink characters are the only reason to make me lean to thinking the Rule As Intended is that enemy units have a choice in combat to attack either.
I would say go over both scenarios with your gaming group and play however you agree to play until the first FAQ states the 'correct' way.