The people at aqualand aren't really experienced at long term mormyrid and knifefish care so I am taking their article with a grain of salt. On MFK they told me when they don't get a long they chase eachother so if there were problems I could break it up.
When I had my FW baby whale it died after like 4 months. I read on some sites that you should not put more than one in a tank because of electronavigational problems, but then I read that they all have their own frequency and should be in groups. I think it would have been much happier in a group looking back on it.