What is the unmodified armor save of an oldblood on carnosaur (before you add light armor and shield, etc). Both the carno and the old blood say 4+ scaly skin. Would it then be 3+ because of the mount?
monstrous mounts do not add +1 to the armor save of the rider but have their own armor saves. the oldblood has a scaly skin save and thats it, the monstrous mount doesn't modify it as a cold one would
Yep, although you always at least add light armor for 3+, then probably normal or enchanted shield on top of that.