Melichthys triggers and the niger can both be kept in groups. Most lionfish, morays, several species of tangs (my favourite being the convict; others include the regal and yellow), batfish,
Monodactylus, dwarf angels, Fairy, Flasher and
Halichoeres wrasses... the list goes on. Really the options for grouping fish, especially in a 6' x 2', goes worlds beyond
Chromis and
Anthias, and though they are beautiful fish they wouldn't be my first choice.
As for the other question, if the niger trigger wanted to eat the darter (it was a barred darter) it would have done so after day one. The size difference was quite significant, but I added the niger full knowing that they are docile. The pinktailed, the other trigger I owned then, was a "kindly dictator"; he was the top dog, but didn't touch any of the other fish, including two blennies and two clownifsh in addition to the darter. He did, however, defend himself against the niger when he attacked, occasionally leaving bite marks, especially in the niger's tail. Keep in mind these two triggers were both several times the size of the other fish.
Read
my profile of pinktaileds in the TFF SW profiles section, if interested. Was and still is my choice for best all-round aquarium fish.