Big fish and small fish don’t mix, generally if a small fish will fit in a larger fish’ mouth then it will eventually, even if the larger fish didn’t intend to!
Diet is extremely important and starving a certain type of fish of the diet they need because you can’t give it to others in the tank is not right and will lead to illness. For example keeping clown loach with malawis will work but you will end up starving the clown loach of their favoured food (blood worm) because if you feed malawis blood worm they will bloat and die.
Water parameters vary greatly, so if we take discus who require a very low acidic pH and compare to say Frontosa from Lake Tanganyika who need a very high alkaline pH, we won’t be able to cater for both so at least one of the fish would suffer from relative poor water conditions.
Keeping aggressive fish with placid fish is not best practice as the placid fish will not be able to relax and are going to be prone to stress and contracting stress related diseases.
Ref: https://www.tropicalfishsite.com/why-is-the-compatibility-of-tropical-fish-so-important/