I've kept them with bluetongues and bearded dragons, I was very careful at first but suprisingly never had any problems. I stuck one in with Varanus gilleni for a while to see how they'd react, the goannas were very inquisitive and kept investigating the jacky, the jacky was very defensive, opening his mouth threateningly, but showed now aggression, the gilleni were completely indifferent to his display. I left them together for a couple of hours, constantly supervised. If you can help it, I'd recommend not mixing them, but sometimes it's possible. A friend of mine had a beardy kill a jacky, so it's definately not always going to be 100% safe, but then again I know of beardies, bluetongues, cunninghams, ackies and many others (including jackies) killing same sized conspecifics, so risks of aggression and injury are probably comparable to keeping jackies with jackies. I'd imagine they'd be okay with turtles as far as aggression goes, but you'd have to be careful of cage design to ensure it's appropriate for both.