Thanks all for the replies. I pretty much agree with everything said, but would still be very interested to read any studies along the line of.
This same question has been asked on other forums, the answers are the same as above, there don't seem to be any scientific reason for 6. If I were a lfs I would recommend 8-10.
If anyone finds any studies in the future on this, please remember me!
I've found black widow tetras to be the same. I started with 5, they were fine, then they killed off the smaller one after I upgraded their tank
, I brought 6 more, lost another two to aggression, brought another 5, now theres almost no bickering (minus breeding squabbles 
) at all, and a total of 13 minnows. In the smaller group they'd nip the other fish as well, which is not good when they startle a large male SD! In the larger group they leave everyone alone. Also a very underrated fish, I have a mix of white ones and black ones and they look BEAUTIFUL playing among themselves, the way they shimmer when light hits them... Amazing. 
