I agree that they are one of the most aggressive fish, I've seen the damage they can do to even their own kind. In my case he gets along great with everything in the tank. He's been living with 3 rocket gar since January when it was just him at 2.75 inches and one gar at about 2". Now there are three gar at about 6-7 inches each, and he's got to be in the 9-10" range.
I've found that he's temperamental. If I mess with him, (clean the tank, move some decs, try to catch the pleco), he will mess with the gars. But he waits until the lights are out. He just chases them around like he's harassing them. I think he somehow knows the splashing keeps me awake. He has never done any worse than nip the fin of the biggest gar, twice in 9 monthes. Only once he forced a gar to evacuate. Good thing they're tough, I just tossed him back in the tank... He hardly even eats the feeders that are in there for the gars, he just eats the sinking pellets that I give him.
I'd say it totally depends on the fish in question.