I once had 3 ST and 1 NGT. Two ST were 11" and the NGT and another ST was 8" They also beat the crap out of each other which resulted to cloudy eyes due to rubbing to each other's bodies.
My guess would be that in both cases the NGT was showing dominate tendencies. The NGT is the most aggressive Dat, and probably tried to assert itself as "boss". The larger ST's probably put them in their place.
Whatever the reason, 7 ST's would look great together.
NGT will beat the crap out of any tiger close to its size with the exception of thin bars
once NGT get to 8-10inch the turn crazy and don't tolerate any tigers
i have a 18 inch ST with a 5inch NGT no problems
i kept 1 x st and 1 x ngt in the same tank for ages they were best mates always shoaling together the 1 day the ST beat crap out of the NGT so bad i had to remove the NGT
when i put the NGT back into the tank he beat up 7 x 12inch ST nothing would mess with the NGT
don't underestimate the power and aggression of a NGT
I've had it go both ways. I've had NGT's that were submissive to the other dats, and some that were dominant to other bigger dats. It depends on the personality of the individual fish but from my own experience, I found the latter to be more true. There is only one way to find out. Just keep an eye out on them because they can get quite brutish and nasty. GL