I had a Golden Wonder Killi in a 10 gallon temporarily amongst typical tropical fish like neon tetras. I can tell you from my experience that I never saw it do anything aggressive whatsoever, but with a mouth as big as theirs it was just going to be a matter of time before it got bit enough to fit a neon into its mouth which is why i moved it to a tank with bigger fish. I'd say its not that theyre aggressive, its just that they have big mouths and will eat what they can fit in, just like a molly or a swordtail isnt aggressive but will eat any babies they can fit in their mouths.