I am going to attempt a list here of considerations one should keep in mind when selecting stock, especially in the starting stage of building an aquarium.
1. Compatibility (Fin nipping etc.) (Here tank size, number two, goes along way with some species)
2. Tank size.
3. Water Chemistry (Soft water fish should not be mixed with hard water fish)
4. Temperature compatibility
5. Adult size, (this goes hand in hand with number two.)
6. Shoaling fish / Schooling fish. (This is important because some fish have to have a sufficient number in size to feel safe and not stressed)
There could be other factors to consider that I have not thought of, but a person should do exhaustive research on stocking using the above to help create a healthy community aquarium.