I don't know about the Kaudern Cardinals (which I want), but I do have the PJ's and they are some of the most docile fish I've ever seen.
They're not territorial at all in the sense that other fish are, and they're not nippy at all to newcomers in the school/group.
My experience is pretty much the same as what Fosters and Smith have to say:
Originally Posted by Fosters and Smith
As a group, they will establish a strict hierarchy without aggressiveness.
PS: I plan on adding some of the Kaudern Cardinals later and I'll definitely post back how they all get along... I'm really hoping since the Kauderns are such natural schoolers that they'll all school together with the PJ's out of fear when they first get added to an established, multi-species tank.
*Crosses fingers and toes*