As far as Idols and tangs, they live together in the wild.
In the tank, they can live as well. In my 400G tank, I had a pair of Naso's over 12" long, Blue hippo tang, three yellow tangs, and purple tang along with my 8" idol. No problems what so ever.
There were also alot of other fish in there as well, three angles, some wrases and gobys and a few others.
As for sponges, like paul said, the are pickey as to which ones they eat. I have a bunch of hitch hicker sponges growing in my tank. There are only two that I ever saw the idol eat and once he became accustom to eating the pellets and nori I put in the tank, he left those alone as well. As far as feeding, I use a variety of sinking pellet food. I buy a little of everything and mix it all up in a big tub. I also feed a sheet of nori per day.
AS you guys know, I lost my fish due to suffication when my pumps went out. But up until that time, my fish were doing great. Idol was going on almost 4 years in my tank. I had gotten him from a RC member.