The theory that I have been working with is that the different choirs are not mutually exclusive. I don’t have any references, or any idea if someone more holy than me held that theory, but i’d be interested to know
A note of caution to all on this thread. Angels are “in vogue” right now, and so there is a plethora of information about them, online and at the bookstore. Most of this is new-age astrological garbage. Even the Wiki-article above has a rather low percentage of approved Church teaching. Every Tom, Dick, and Madame Clara feels qualified to publish a book about angels and make a quick buck. I strongly recommend, if you truly feel called to learn more about angels, visit a Catholic bookstore, or research the author/pick an author you know to be orthodox at the secular bookstore. Peter Kreeft and Joan Caroll Cruz are good, knowledgeable, careful authors, I have Kreeft’s “Angels and Demons”, I don’t have Cruz’s angel-related books, but 2 others of hers, and I was quite edified and informed by them. Tan and the Daughters of St. Paul have published novel-ish-length books and novena booklets, and are very good publishers.