Understanding the factors that contribute to stress, will eliminate or reduce unnecessary risks. Stability in the water column and a balanced environment are as important as eliminating some other potential stress causing factors. Compatibility is the key. Aggressive fish chasing others, naturally or as part of a mating ritual or territorial defense. Smaller species without adequate hiding places feel unsafe among bigger fish.
Stress also occurs when feeding if the fish have to compete for food.
Needless to say they will always compete. The thing to remember is to distribute the food evenly throughout the water surface. Avoid spot feeding. The rule of thumb is about 1 inch of fish per 2 Gallons of water. This guideline should only be used in order to get a sense of the limitations of your tank.