Not an expert but i have about 14 angels in my tank. If possible, buy from small to big. But i understand some rare angels u cant get to choose the size. What i do is put them in the main tank with divider between the new angel and the current ones. This is to let the new angel settle down and start feeding and to prevent from been bully. If the new angel is bigger than the current one, once it is feeding, u can release to mix with the rest.
This was taught to me by TFC