I have a yellow-headed/fimbriated moray along with an undulate trigger, and they both get along extremely well. They steal food from each other all the time, but nobody seems to mind. They like to hunt together, and they'll even chill in the same cave for hours. BUT, they both go ape ---- when I throw in some feeders.
The fimbriated eel isn't exactly that mean, but the undulate is another story. The relationship with the eel notwithstanding, the trigger can (and usually is) absolutely vicious.
Just today, for example, I tried adding a large louti cherry grouper. Previously, I had another grouper which the undulate completely ignored. The only problem was that it could - and did - fit inside the eel's mouth. On my second attempt, I bought a grouper large enough for the eel to
not swallow... but I guess the trigger had a problem with this particular grouper (or its size). It was dead, its back half completely ripped off, within 45 seconds. Poor fish. Poor wallet.
In fact, anything moving at all would piss off the trigger. It's quite amusing to watch the trigger attack my mag-float, and equally entertaining to watch it shred crabs to pieces, and snap mollies in half with a single bite.