Yeah, but, you still wound on a 2+, if you throw 6 attacks, you'll hit with 3, wound with 2.5, and kill 1.25 knights, skinks get 7 attacks, 2.333 hits, 1555 wounds, and .259 kills, so on the charge you kill 1.59 knights in total. (average) If you get the flank, even with reduced skink attacks, you should win by a lot. If you charge them then with a unit of 6 knights you'll have....
4.41 knights making 9.82 attacks, 6.5466 hits, 5.45557 wounds, and no saves
They end up killing 3.8655 more guys then you, if they exclusively attack skinks. This means that with your 3 ranks and an outnumber, you win by .13444. 2 kroxigors and 22 skinks will in fact beat knights on the charge, though it is a very marginal statistical advantage. If they charge you.... be sad. Thankfully, that formation of knights I pointed up costs almost 300. A more fair matchup by points cost would be 3 kroxigor and 24 skinks, which on the charge could really mess these guys up... it might even be able to mess up the knights on the counter so... math time.
6 knights make 13 attacks, hitting 8.666667 times wounding 5.777 times on krox or 7.222 times on skinks,
dealing 4.814 krox wounds or 7.222 skink wounds. This prompts the same krox situation as above for 1.59 knights above with the krox being targeted, and... 1.875 knights killed if they swing on the skinks, with .872 being the number killed using abstract numbers of attacks.
If they hit krox, the CR for the knights is... 3.224 more kills then you (or 4.227 using abstract attack numbers), and 5.347 more then you if they hit skinks. If they hit the krox you lose by .227, if they hit skinks you lost by 1.247.
Also a skink unit 6 wide will kil .2222 knights with their stand and shoot reaction, which is enough to make the difference in the value less then a tenth of a percent if they choose to hit the krox. The fact of the matter is..... skinks and Krox may not beat Chaos knights every time, but they will beat them often enough, and will beat them REAL BAD on the charge, whereas chaos knights only beat you a little bit when they charge you. For a core unit fighting special, this is not bad at all.
Unfortunately, this math assumes that they have no marks, marks would screw you right up, most notably the mark of nurgle, which would make the kroxigor hit 50% less, or the mark of Khorne, which would make them hit more or less 50% more. (but for the cost of some of those marks....)