Typed in a response at work but then all day never got a chance to finish it and press submit. Argh! Hate days like that.
Anyhow, the short version to what I want to say is that I have two C. ulietensis, aka Pacific double-saddle back, or false falcula butterflies. Both C. ulietensis and the similar C. falcula (aka "true falcula" - the black patches are smaller on falcula) have a reasonable reputation for being majano munchers, reasonable adaptability, and a somewhat reef safe reputation. The problem is that the flipside to "somewhat reef safe" means "somewhat not reef safe" and you'll not really know what else could be at risk in your tank until you've tried them and find out. They could be model citizens but they might end up not being able to resist certain temptations and you can't really compensate for a learned behaviour with extra feeding.
I was warned beforehand that "they eat everything" and I thought. .. well, surely not EVERYTHING. But 8 month in with them I can say it's actually darn close to everything. Softeys, stoneys (LPS and SPS), gorgonians, clams, anemones .. no category is immune. For the most part they leave most things alone but once they've decided one individual is food, that's it, it's food, and they won't leave it alone until it's gone (perished or removed). The latest thing is I've had a couple clams let go of their rock (like clams do) and the butterflies discovered that the byssal gland is pretty tasty.
So the upright clams are fine but a couple in the past month did topple over and I found them too late to save.
So it's a VERY carefully measured choice. They can be pretty destructive. Which is too bad, because they are VERY nifty fish.