I always have otos in my discus tank and cories and rummies and amanos and have never had them go after another fish. When i do water changes the amanos scurry all around the tank all disturbed, sometimes right in the faces of the discus and the discus pay them no mind at all. I kept albinos in with them for a couple years, moved them out to get smaller false jullies and still have no problem. But my cories never exhibited any fear of the discus. The amanos stay under cover as do the rummies. The rummies live in the plant thicket full time. And I have a few black neons that are not afraid and swim out in the open freely. I think that a lot of smaller fish cannot overcome their fear of bigger fish possibly eating them. It's in their genetic makeup, part of their survival. So if they act frightened, they are sending all the wrong signals in that tank.
You should see how fast an ember tetra can eat a newly hatched baby pleco!