I've kept 2 Idols for about 6 years and 2 for over 10 years; prior to moving after Katrina. All four of these acclimated well and will /would eat anything. I know that sponge is a big part of their diet; but they are true omnivores and the only sponge my present 2 get are occasional bits in frozen Angel foods. The 2 I had for 10+ years never got any. If its available to you, great. Like some sponge-eating Angels; i think they can be weaned onto a healthy, varied diet. Fresh clams are a delicacy for them and usually the first food I offer them. IMO & IME, smaller Idols are easier to acclimate to a captive diet, the same is true of sponge eating Angels. I assume the younger fish haven't developed a rigid diet yet. Again, IMO & IME, the big problem with these fish is they often refuse to eat anything. Once eating and comfortable, I haven't found anything they won't eat. They really need a variety of foods and vitamins, like all fish. greens, meaty stuff, seafood (shrimp, clams, scallops, etc.). I agree with the need to feed them often.
A few things I would consider before buying (IMO & IME): 1)A minimum 8', fairly tall tank. Maybe a 240. 2) A min 55 gal QT and the willingness to QT and acclimate for 8 weeks with no other fish in the QT other than the Idol (or 2). 3) A DT without fish that will harass it; like aggressive triggers, etc. 4.) I would never buy one from anywhere other than Hawaii or Australia; but Tahitian fish may do well too. 4.) No matter who does what, many of these fish simply don't eat. If that bothers you, it may not be a wise purchase.