My personal experience points otherwise as one of our jaus when it was 2' bit off a pectoral spine on a 3' and 25 year old Oxydoras niger aka ripsaw catfish. But people's experience can and do vary wildly.
Still, jau are a moody fish and indeed aggressive to their tank mates, when they enter into their territory or their liar / corner. Mine two 90% of time tolerate their tank mates entering their space but then 10% of the time they snap at and bite RTC, pacu, TSNxRTC, piraiba, paroon sharks, Distichodus, etc. I won't go into how they fight with each other, two 40" jaus going at each other... The key is for the tank mates to learn and keep away. If they fail, they will be crippled.
I'd not place timid fish such as large Doradidae, IDS, even channel catfish with jau personally.
They are a hard, hard fish to house in a community.
More is in my thread on our jaus.