I'm far from an expert in this area and have never kept them. However, I was planning a 180 lake tang tank a year ago before I started saltwater. If memory serves me, this is the acceptable size for a group of benthos.
I'd recommend in putting Benthochromis sp. in no less than a 6' tank with plenty of open space and dim lighting. As with most things, the bigger size tank, the better. I have a group in a 7' species tank, and it seems as though they are cramped.