I think the recommendation for getting a group comes from their generally not exhibiting natural shoaling behavior in small groups, and perhaps this leading to them potentially feeling more stressed. But, if the choice is between upsetting a balance that's working and getting the group up to six, I'd probably stay with the status quo.
They seem pretty happy. They hang out together and even feed with the other fish at feedings. And, they are always scurrying around with their searching and sifting