Actually, all four species need a few more, some seriously so. As you've asked the question while I was typing, I'll try to answer it.
Shoaling fish species live in groups of doz\ens to hundreds, depending upon the species. The individual fish "expect" this because it is programmed into their DNA. When denied the group (shoal) they will be stressed. Stress is directly responsible for 95% of all aquarium fish disease, plus it generally weakens the fish, and leads to other internal issues. The fish, putting it simply, has a significantly more difficult life, working harder (and using more energy) just in an attempt to maintain its necessary life processes. It will never live to the average expected lifespan for the species because of this. But usually along the way it succumbs to some other issue which it would otherwise be able to easily deal with, but now cannot because it is weakened.
Everyone asks for minimum numbers, and sometimes that may be a necessary concern, but the fact is that the more of the species there are, the more "relaxed" the fish will be, and that is a bonus for the fish and the aquarist. If space does not permit more adequate numbers, it may be best to select another species, depending upon the species involved.
Corydoras must have at least five or six, but a group of 12 for example will always mean "happier" fish because they will be less anxious and thus healthier. Cories are very social; I have 41 wild caught cories, representing 12 species, in my tank, and they have been together now for a decade or more, except for the few fry that have appeared now and then. They chum around across species and frequently spawn. They are healthy, plainly put.
Black Phantom is another species that needs more, here at least 9-12. This is not a very active species, but it will be out and about more with a larger group. I would probably aim for 15 in a 300 liter tank. You need plants, they spend most of their time among plants where it is darker (they have something of a light phobia, common to most characins actually).
The Colombian Tetra can get a bit fin nippy in small groups, and six is small here. Another 3-4 would probably avoid any chance of this, though one can never be certain.
The barb should be kept in a group of 8 or more, or it may become shy and stressed.
Increasing the fish as above would be priority one, and then consider possible additional species. Scientific studies on this particular aspect of fish are few to date, but those that have looked into it have found that the individual fish tend to show increased aggression with smaller numbers; aggressive fish became more so, and peaceful fiswh like neon tetras became mildly aggressive. Clearly there is a profound effect on the fish, and it uses the few means it has to deal with the frustration and stress.