Curving off your Jardini's aggression would depend on several factors. I only speak from my experience but hope it'll be helpful. My 14" jardini is living with a temensis, silver arowana, and two stingrays. It also tolerates a 13" black arowana.
1. Tank space. Bigger the better.
When I put my Jardini into the 135g, it would go after its tankmates. Once it assumes superiority, it'll relentlessly go after other fish. It does best in the 210g. It doesn't bother any other fish.
2. Don't let it become dominent.
Always mix it with at least one large or aggressive predator that it can't domineer over. I do it with a 20"+ Silver and have tamed the jar with a nice Mr. Midas.
3. Frequent disciplining.
When the jar reached about 11", it began attacking the bigger silver which was about 18" back then. Whenever beating episodes occurred, I removed it to a spare tank. I repeated this process until it stopped beating its bigger tankmate. Stragely, it doesn't attack any other fish anymore, when it is in my 210g.