When Magnum formed, it was largely an economic venture, but there were photographic rules. I understand that Cartier-Bresson was adamant about acceptable focal lengths. Both had great impact on photography, and I think both could be considered movements in hindsight. Plus, Magnum fought the economic battle for all photographers, and now we own our photos.
Good point bringing in the idea that Magnum also was a movement which defined the qualities expected from its members.