Unless something has changed since the post below, matching with the legacy or TMDB agents is ok if the library is using Plex Movie as its primary agent; matching with Plex Movie is not ok if the library uses the legacy or TMDB agents as its primary.
So, what you’re trying to do should be possible; why it isn’t returning the proper poster I’m not sure. However, if you fix match and leave the agent as Plex Movie, does it not give you the option for “Intergalactic Thanksgiving or Please Don’t Eat the Planet?” It does in my case, it’s just not the first match. It pulls the poster from TMDB and the rest of the metadata appears to be correct.