I just added a H. melanurus a few weeks ago with my existing H. leucoxanthus. and they are doing just fine together. The melanurus is larger than the leuc. and I have seen it show signs of aggression about once a week, but its nothing really that big, more like they got to close to one another. It is usually just a quick jerk towards the yellow and the yellow darts off. I have no worries about the two of them being real problems, agression is almost non-existant. Just thought I would update this thread for anyone interested.