So does that mean if 2 different fish from 2 different parts of the world who live live in similar conditions were put in the same tank then it would be ok?your last point "stand the same conditions" is for me the key
many things impact on conditions and as I said we cant ask the fish if they are "Happy"
You can't ask a dog if it is happy but can usually tell if it wags its tail. With a fish you can normally tell if it isnt happy by colour, how active it is, if it feeds etc and usually if it is belly up floating around the tank then I normally think that fish is not happy.