Hi, I'll help however I can. I'm curious, how big of a tank is your trumpet fish in at present? Trumpets top out at just over 30 inches and will eat any other fish they are able to. The trick for finding something compatible will be to find something large enough to not be food and peaceful enough to not bother the trumpetfish... some of your angelfish might be possibilties, also some of the larger tangs (like the unicorn tang). Please be sure there is plenty of space for all fish and that filtration is good. I didn't see a whole lot of info posted on them online, and some of what I saw was inaccurate.
I don't know what the life span is on them, but I do know it is potentially very long (20 - 30+ yrs) if they are housed properly, which can be hard to do because of their size.
The trumpet fish is a predator, and it stalks it's prey. Trumpets tend to swim in "open spaces" around reefs, stalking other fish for food and patroling the reef. They are also found in deeper waters of the reefs, if that helps. Do some studying about reefs and their makeup at depths of 15 - 30 ft and you'll find the natrual habitat of the trumpet fish.
Here is a bit of reading and referral to other reference materials that can help get you started: http://www.amonline.net.au/FISHES/fi...sh/achinen.htm
Hope this helps. Can you post a pic for us to see?