They are pretty easy to keep from what I have experienced, I had a cowfish for a while and he was eating well and cruising around. Very interesting, hovering kind of fish. He would always peer out from behind rocks as he hovered up and down.
However as scuba said they may not be the best aquarium fish in the world because of two main reasons, they can excrete poisons when they get freaked out and also they tend to get sort of big... like 18 inches. LOL
They have a bony exoskeleton with all of the guts inside and this is why they look like skin stretched over a box.
I traded mine to dude at LFS
for some giant snails and a sailfin blenny. He currently has the cowfish in a display tank at a tavern in town. I traded him in because I wanted to eliminate the risk of something scaring the crap out of it and having it poison the tank.
If you choose to get one just read up on 'em and remember that they get big. They are very interesting little guys.
Here is one of many links on these critters:
(edited) Oh. and as far as getting along with your inhabitants I would bet that they would just ignore the cowfish. You're best to research that particular issue of course with your particular fish but.... my fish acted like he wasn't even there. In that tank I have yellow tang, and feisty damsels too.