The "only one angel per tank" rule applies to large angels rather then dwarf angels, as they have different social habits in the wild. In the wild dwarf angels form harems, consisting of 1 dominate male, ruling several females. The problem with adding more then one dwarf angel per tank, is that you simply don't know what you're adding. Add a dominate male Lemonpeel to a 40gal with a dominate male Flame, and of course they won't get along. The other problem is that even if you were able to successfully collect the entire harem, because of the change of scenery, the social-structure of the harem, will be reset, and whoever is the most dominate, wins. If you take proper steps for introduction, multiple angels can be kept in the same tank (true for both large and dwarf angels, in that case).
Dwarf angels are herbivore-biased omnivores. While dwarf angels do not infact require sponge in their diet, sponge-based foods are a pretty good food anyways, and definitely won't hurt to feed. Emerald Entree and seaweed sheets for the veggies, and it's important to get meaty foods in there as well, mysis, finely chopped scallops, and fresh clam are all great foods.
