I have not kept more than one species together, so I can't really comment much - I know that Australians and boesemanis can interbreed which makes me think they may school, but the greater difference in species, the less likely.
I had four boesemanis and still had some aggression issues. It's very important to have lots of hiding spaces and adequate swimming room. What size is your tank?
The six per group rule is typically applied to a single species.