My two 10" monocs rarely bother any of my other fish in the tank. They do, however, squabble with each other. I'm pretty sure I have two males and after they eat, the larger individual spends a lot of time chasing the slightly smaller one around. I'm hoping to pick up 2 or 3 more in the next couple of months so that the one won't get picked on so much.
Every once in a while they'll nudge one of the oscars they share the tank with, but the slightly smaller oscars usually call their bluffs and chase them away.
For the most part, if it clearly can't fit in their mouth, they usually leave it alone.