First can he join at all? Second, does he interfere with Kroxigor stomps or the monstrous infantry supporting attacks from the second rank?
He can join but he would not get a look out sir as he is not the same type of model (he is a normal infantry model). He does rank up with the unit though. I would say that it would intefere with supporting attacks if you were more than two ranks deep. If I were running him with the unit, I would put him to the right or left of the unit with a single model behind him as the supporting attacks are 3 for monstrous infantry regardless of what the front rank attacks are.
Oddly enough, the PoS would lose unbreakable while with the unit. While the rules for stubborn allow a stubborn character to make the unit stubborn, the same does not apply for unbreakable.
PoS would have to rank up within unit as option to go to side only applies to non-fitting models (I think, can't check now), he is on 40x40 base if I am thinking of correct character. Would only affect stomp in that he would have to be in front rank thus preventing a krox with stomp to occupy position. Support attacks are calculated by model not by rank so a krox behind PoS would be the model behind a model eligible to fight normally so would get 3 support attacks, would not get stomp (like other 2nd rank krox) as not in base to base contact.