That is a good question. We know some general things about certain fish species related to behaviour, but each fish can to a limited extent be unique, and at the same time environmental factors can affect fish in different ways.
The reason angelfish and gourami should not be together is due to their very similar temperaments; the males of both species are very territorial, and this usually mean aggressive--when the mood strikes them. It is best not to combine them because the dominant angel or the dominant male gourami might one day decide he is fed up with those other fish invading "his" territory, and off he goes. Result can at best be very stressed fish, or at worst, dead fish.
Some have tanks with both fish and no apparent problems. Sometimes this lasts for a while, then game over. Others put the two species in a tank and one kills the other within days.
Another quite unrelated problem with dwarf gourami is their health. You can read about this in our profile [click the shaded name]. Many do not recommend even buying dwarf gourami unless you know exactly where they came from, by which I mean, the breeder, not the store.