The issue is not size, at least not immediately, but numbers. I am assuming from your posts that you may not be aware of the shoaling issue, so I will explain as best as I can.
Freshwater fish that are a shoaling species must be in a group from day one. This has consequences, and depending upon the species these may be severe. This shoaling need is programmed into the fish's DNA, and it applies to all characins (tetras, pencilfish, hatchetfish), rainbowfish, cyprinids (barbs, rasboras, danios, loaches), some catfish (Corydoras for one). Some species just need the safety of having the shoal, while others may have hierarchical issues. When shoaling fish are denied a group, it has severe consequences which may not even be noticed for weeks, but this is causing stress which slowly weakens the fish. Down the road, they are more likely to succumb to health problems, aggression (or the opposite), and always will have a premature death long before the normal lifespan. Please read the blue and (especially) green statements in my signature block, they are relevant here.
Loaches are highly social fish; in their habitat they live in groups and they develop an hierarchy within that group. Scientific understanding has determined that five is the minimum for loach groups. When two, three or usually four of the species are together, the hierarchy does not develop the way it should (with more) and while this may not seem problematic it can quickly become so. The fish usually find one to bully when there are too few to supply an adequate shoal, and this can go unnoticed until the fish becomes withdrawn and dies. It is quite simply cruel to the fish to deny them what they "expect."
