I've had my african lungfish in a tank with a ropefish, black ghost knife, a pleco, and a catfish, since august and he hasn't made any attempts on them. Ate some tetras but that was to be expected haha.
My roomie has a silver arowana and had a gar before he sold it. Lungfish was in the tank for 4 days and didn't get in a fight. He did however seem really stressed even after 4 days in the tank. From how they acted, seems like the arowana and lungfish wouldn't clash...when the lungfish comes up for air arowana just swam away. The gar liked to swim in the middle and bottom and seemed more aggressive than my lungfish, so I'm guessing they might get into it on a long enough time frame. But like I said, my lungfish was only in his tank for 4 days so it's hard to say.