Generally fish keepers classify fish into 1 of 3 categories, 'Community Fish', 'Cichlids', or 'on their own', and although there is some justification for this harsh division it is not the whole story. To be just a little more accurate we could subdivide the Community category in small, medium and large, and the Cichlid category in 'Lake Cichlids' and 'Others'. Better, but still misleading.
The truth is, its impractical to attribute generalisations to whole families of fish, their behaviour is just too varied.
I'll give you a extreme example: most people would classify Tetra's as pretty benign fish, and this is true for Neon's, Cardinals, Glowlights etc etc, but the Silver Dollar (Metynnis hypsauchen) and the Red-bellied Piranha (Pygocentrus nattereri) are also Tetras.
True they are both bigger species, but what about the Serpae Tetra (Hyphessobrycon eques), only 4cm, but can rival any Tiger Barb when it comes to fin nipping, as many a fish keeper has discovered to their cost.
Aggression in any aquarium, community or otherwise, is the underlying cause of many of the common afflictions amongst captive fish. Frayed fins, or even dead fish, may be a blatantly obvious sign of bullying, but is by no means the only problem which it causes.
Even if the aggression is only posturing and never actually becomes physical, the stress caused to other fish can be just as bad. Timid species hide and may refuse to eat, while the general stress usually leads to an outbreak of one of the opportunistic diseases. Even a single unsuitable fish can disrupt an entire tank so choose your inhabitants with care.
So how does one go about selecting compatible tank mates? The only solution relies on the experience of others until you have opportunity to learn for yourself, and this is where your local fish shop comes in.
We can't speak for other outlets, but at Grosvenor Tropicals we do have a little experience which we are willing to share. If you already know the characteristics of your chosen subject then great, if not, please ask. We will do our best to steer you on the right path!