My pair are pretty good with other fish else protecting fry as mentioned above also. Sometimes I do find the odd baby con headless which I accuses the male jag off as his always hiding after a fish is headless but have never seen it mind you these fish are 1-2 inch and my male jag is 12-14 inch at a guess. The male has almost killed the female 5 times, he likes to beat her up but other fish I have never had an issue else smaller but thats kind of expected, Most of the smaller fish are fry that I have missed. He has killed my green terror 5 inch, terrors fault for trying to eat the fry apart from that his a good boy. My oscar pair will flare at him and all he does is gives them a little kiss or lip rub.
I have 2 pairs of baby jags in my 550G they are 5-6 inch + for the males the females are smaller. They dont try to fight just sit there and dig nests all day and eat they will chase fish away double - triple there size but the second another fish turns around the male will run, they do fight with the dovii pairs in the tank 2-3 I have lost count too many doviis every tank seems to have one lol but its rarely a lip locking mind you I have 2ft fish in this tank and the avg fish size is 12inch + so using these 2 jags may not the best example there are smaller fish and fish the same size as them in the tank too but its not the best example...
All in all I find jags are talked up a bit too much, sure you get the odd one thats a killer but all in all they flare lip lock and run away else the fish is smaller etc etc.