I believe it is because Sev's are not as aggro and confrontational (most of the time) as JDs/GTs, So you end up with a dominant fish (the GT/ EBJD/ JD) and the sub-domninant fish (Severum). If you try a GT and a EBJD they may both try to be the dominant fish and end up doing damage to each other in the long run.
But it depends on the nature of the JD/GT. In my opinion I think a EBJD and Green Terror would do fine together, given enough room, hiding spot, etc.
I am pretty sure there are a number of members that keep ebjd and GTs together.