Ok, first off no traditional Christian faith that I’m aware of accepts that angels are the souls of the deceased or vice versa. The Catholic Church has stated before that angels are eternal beings, created by God on the first day, so if you’re looking for canon there it is.
Most of the confusion surrounding angels and the dead comes from the Middle Ages, a time when public interest in angels was skyrocketing and people were dying in droves between the Plagues and other facts of Middle Age life. I couldn’t give you an exact “Joe Smith said this” definition of it, but the general sense of things was: people were dying by the cartload, angels were named as Saints (as Chronos pointed out), dead humans were named as Saints, angels live in Heaven, good dead people go to Heaven, people wanted some solace of their dead loved ones watching over them, a few Biblical stories have a dead person watching over someone else, guardian angels watch over people… etc into one big mess until the whole notion that dead people = angels emerged. Keeping in mind that the average person in the Middle Ages knew as much about Catholicism as they picked up from looking at the stained glass windows while listening to the Latin mass.
Anyway, the Church was never too pleased with this whole outlook and has said that it ain’t so in the past. These days, the Church seems to try to downplay the entire notion of angels at all given the New Age angel attention (and the Church doesn’t like facets of the faith that take the spotlight off Jesus – they’ve gone against Mary worship in the past as well).
(None of this is meant to insult the Catholic Church, just make the point that they are aware of the dead=angels idea and have spoken against it)