They would beat the heck out of each other until you had two, and then they would most likely fight. The proper answer is not "don't keep 2 in a small tank" but don't keep multiple dwarf angels in a small tank. In most cases, it will not work.
Thats an expensive rumble waiting to happen. Most people cannot put two together unless they are a male/female pair and even then it takes a little effort. Multi species in a 55 is just crazy. Save the fish and dont try it.
i have kept together with no problems but most of the time they will fight with each other. if you want to keep more you just have to try and see what happeneds, but this one LFS always sells groups of dwarf angels together as a set and the owner says to introduce 3 at the sametime of different size and color and they might get along
if u want more info you can email me joker18264@yahoo.com