I don't think they need such a large tank either, at least not one, a trio would need a much bigger tank. I collected one in 2006 and he is in my 75 mixed reef, no hard corals but lots of soft coral and inverts. I keep mine at around 70, and he (or she probabaly at first) has grown from 3 1/2 inches to over 6 inches.
As far as diet, there are other fish in the hobby that eat sponges almost exclusivy that do fine in tanks, queen angels and french angels from here are sponge eaters where 90% of their diet in nature is sponge. The main thing to remember is the people who are successful have a few things in common, the temps are low, low to middle 70's, mixed and varied diet, and well established tanks with good water quality.