In my experience, Congo tetras (P. interruptus) are not aggressive at all. Even full grown ones (3+ inches) are aggressive eaters, but not aggressive among themselves, nor toward other small fish, nor nippy even towards fish which have long, flowing fins (i.e, angelfish fin extensions). I have a small group in a 125g and they are not aggressive at all.
Exodon, on the other hand, are aggressive, and nippy. I admit I have never kept them myself, but I have seen them in action.