I have a 9" pearsei in with an equal size Green Terror, Oscar, and IT Datnoid. The pearsei is always picking scales off the side of the oscar with its crazy teeth.
My 7" pearsi is with some smaller 4" more aggressive without a problem. I wouldnt compare their temperament to say a severum of the same size but they are still pushover to most tank mates. Ide say in a big enough tank your good.
The fish size vs mouth size rule is always a good one. However, in my experience I've found that my larger fish tend to do better with much smaller fish over fish closer to their own size. I believe it is because they don't perceive them as a threat.