I have added multiple knives to established tanks with a larger knife. Just keep an eye on the little guy when you add him. There seems to be a period when they establish dominance which usually entails some biting. Each time, it lasted 3-4 days and the smaller one did sustain some minor injuries, but then they mellowed out. They live fairly peacefully now with only an occasional confrontation. I still have problems with my royal knife because she is very mean in general depending on her mood. But the clowns seem more peaceful.