It would be fair to say that every fish, not just Armatus, would be best for a species tank only. Even if you put fancy goldfish with angel fish, there is stress and disease threat present among them. You may not notice it, but it doesn't mean it does not exist
Do you realize that Armatus is still new fish to the hobby, compare to some other species? We fish keepers still learn about this specie. How did you come up with the idea that Armatus lives solitary in the open water? Imagine something like that would be as same as imagine of riding a unicorn over the rainbow!