I had two who were in a cold tank. When I switched to tropical they were in the same tank, never had a problem with them being agressive towards other smaller fish, well not that I seen. Never had any drop in numbers either. I was also told by the shop that they are agressive so don't put anything like goldfish in with them, otherwise they would be smashed.
All aside, I think if they start chomping and chasing other fish, remove them.