From what I understand (and correct me if I'm wrong) the PMMM movies are basically condensed versions of the anime (and Rebellion is it's own new story). So, what I'm asking here is, is it necessary to watch the whole anime to get the story, or will I understand it from just watching the movies?

From what I understand, the first two movies are basically just the anime, but condensed, and with slightly better animation. I personally would just watch the anime, because it doesn't have anything cut out of it. You can then go from the last episode of the anime directly to Rebellion.