Characters can join the kroxigors, otherwise they wouldn't make a special rule for flyers and chameleon skinks that say: "Characters cannot join this unit".
At doesn't make any sense in my mind :?, looks to me that there's a loophole in the BRB rules Because I swear there's a rule in there saying that characters can't join units with multi-wound creatures. Besides that, thier 'Charge though Skink Screen' rule (forgot the real name) cannot confer onto the Oldblood that had joined it, thus meaning that the Unit cannot benefit from that rule whilst the Oldblood, or any other character for that sense, is part of that unit.
There are several disadvantages to be had in putting him in that unit (if it is legal rules-wise).
1) He cannot benefit from the 'Look out sir' Special Rule
2a) If the character hasn't got the CotJW, he will slow down the unit to a pathetic 4" move/ 8" charge.
2b) Even if he has got the CotJW, he won't be using it to it's full effect by being slowed down by 3"/6" whilst in the unit and yes, the unit can move it's full distance but cannot charge through the Skink Screen as the character does not benefit from such a rule.
Sorry for the ranting guys it's just wierd to have that pointed out, surely that can't be true. The idea may be a cool one (for that case, it's downright sneaky

), but it is blunted somewhat by the above points which waters-down this idea to a less than viable option.
Still, nice try though
-Kai-Itza-