Some good observations. My comments are geared toward South American/Amazonian habitats which I have researched more deeply, but from what I have seen this applies to much of south and SE Asia.
But first...a point I have made so frequently but it still must be kept in mind because of its extreme importance to fish health: each species of freshwater fish has evolved to "fit" or function (=live well) in a very specific environment. Environment includes the water parameters, habitat conditions, numbers of that individual species, and other species of fish. We are discussing the latter aspect here, but this entire fact is the key to success in aquarium fish. They have to be put in an environment they "expect," because all the above factors are part of their DNA.
When one observes these fish in their natural habitat either first-hand or in (authentic reliable) videos, one notices there are actually very few different species of fish in each distinct freshwater ecosystem. The highest number occurs in the very large rivers, but even here, there are what might be considered mini-ecosystems along the banks, under overhanging forest, or out in the main channel. In the majority of small creeks and streams the number of species will likely be a handful, if even that, and they will generally be of similar size.
We also need to keep in mind the fact that in even the smallest streams, the volume of water and the physical space in terms of distance are significantly beyond what we can provide in any home aquarium. Fish species are not as close together, and any species that feels threatened can quickly move to safety. Plus, there is the fish communication tool...chemical signals released by fish. Pheromones are ready by others of that species, and allomones are read by fish of other species. Packed in the average home aquarium, and especially if partial water changes are not regular and substantive, this creates stress to the fish, unseen externally, but steadily weakening the fish, making it more susceptible to other issues with which it would otherwise more easily deal.
Compatibility of the species included is obviously key. Predatory fish is something many of the smaller species do not normally encounter, and flight is their response. This is denied them in the smaller confines of even a large aquarium, so the stress adds up until it becomes acute and then the fish begins to lose. Compatibility within the species is also important; if the fish is known to have aggressive tendencies when maintained in smaller numbers, then make sure it has a large group. This are normal inherent behaviours that must be recognized and then provision made, or the fish will never be able to live "normally," and will always be weak. Energy spent dealing with even small amounts of stress is energy not being focused on basic living processes that are vital to the life and health of the fish.
