Had a tank set up with several 12"+ cichlids (Oscar, Jag, GT, etc.) and Giant Danios. They all grew up together and were never feed live fish so maybe that helped. The GD made great dither fish.
~5" 3-spot gourami with a ~14" Red Belly Pacu in a 55g (this was when I was a little kid and didn't know better). The gourami was very territorial and would often chase the pacu around the tank. For some reason it ignored the two small O's that I had in the tank though.
Ornate Polypterus and Palmus Polli + 15 black skirt tetras + 4 cherry barbs + 1 Nic Cichlid + 2 Polystigma + 2 livingstonii + 1 Rescue Oscar... in a 29g. Before anyone pulls out there flame guns this tank had 4 HOB filters (most rated for 40g tanks+) large water changes and all fish were upgrade once they out grew that tank... although most of the tetras and barbs "mysteriously" disappeared.