6X4 is probably not bad. In my mind I always imagine these monsters hitting close to the 3ft mark so I always try to think how much room the fish would have to move around. When I was in my teens and before I knew better, I had a TSN in my 6ft 150 gallon tank that was 18 inches wide. It quickly grew longer than the width of the tank so I will always strive to not repeat that mistake.
I think I personally would prefer something 8X4 or larger.
As far as a few black aros in the 6X4 as well as the TSN, I think that might be stuffing it a bit as the black aros will grow large themselves.