My experience --
When I bought my dwarf lion, he was about the size of a quarter. He was put into an established tank with a small clownfish, two engineer gobies about 5" long each, and a toby puffer about 2-3". Oh yeah, and a tiny yellowtail damsel that refused to ever die and survived every little start-up mistake I ever made. He never bothered a single other fish. I fed him frozen krill and brine, and occasionally put some live brine shrimp in there. He grew pretty big, and never harmed a single other fish (though there were times I was a bit nervous if I missed a feeding schedule!).
Had someone else feed my fish and instead of melting the frozen block of krill they just dropped it in the tank. He came up and ate the whole block at once. I don't know what happened next but it can best be described as a complete meltdown of the tank. He started twitching and shaking, and all my other fish started to die. As in, they couldn't swim right, were starting to either float at the top, swim erratically, or just hover at the bottom on their side... like he had poisoned the tank. I managed to get them all out and only lost one fish, the yellowtail damsel.
People told me on here that they don't poison the tank if they die, LFS employees told me the same, but I can tell you that I watched it happen! I guess the moral of the story is... be careful what you feed the (dwarf) lions!
