The article you are referring to deals specifically with Loaches in African Cichlid tanks. They are typically deployed to deal with snails. I agree with the article. African Cichlids are typically kept in a high pH environment. This environment is unsuitable for loaches because they require a much lower pH to thrive. This high pH results in deficiency in osmoregulation and can also damage internal organs.
African Cichlids can also be aggressive toward loaches. This is a lesser issue, but significant. Loaches should be kept with fish that won't directly bother them if they are to thrive. That does not mean that loaches can't be kept with aggressive fish, it simply means that they should be given an environment where they will be free from territorial attacks. I have kept Clown Loaches with SA and CA cichlids without incident, but these are larger specimens. If my cichlids showed aggressiveness toward the loaches, one or the other would removed. Clowns don't have a significant means to defend themselves.
