Dwarf Angels are generally more reef safe than their larger cousins. Large angels tend to eat sponges and polyps etc where as dwaf angels do this less often. As for how they treat each other.. wel this can be problematic. I have had success with a Bi Colour Angel, Coral Beauty and Flame angel all in the same tank but these were introduced at the same time with plenty of hiding places. Its not a good idea to put another angel in a tank with an already established one. They also needs their own space so in a smaller tank i would only recomend that 1 is kept. Lovely fish thouhg, very beautiful and always actively searching the rocks for food etc.