1. Number of parts in hymn playing is four. No, you can choose any number of parts between 1 and 6.
2. Disposition of parts is SATB. No, any voice can be placed in any part, even the soprano.
3. The part in the pedal can only be the bass. No, treat your feet like an extra hand and play any part you want. Even the soprano. Especially the soprano.
4. Rests between the phrases are only for breathing. No, you can add flourishes and runs of any kind between the phrases (just like Bach when he returned from his study with Buxtehude).
5. Registration is principal chorus with mixtures. No, depending on the size of the congregation and the meaning of the particular stanza of the hymn, you can choose a wide variety of combinations from flutes, principals, mutations, and reeds.
6. Articulation is legato. For hymns created after 1800's, yes, it's best to play legato but for earlier hymns - use articulate legato touch (smooth, singing style - not too choppy, emphasize the meter - the alternation of strong and weak beats).
7. Introduction for the hymn can only be the first or the last phrase of the hymn. No, you can create anything you want for an introduction, even a short 3 part fughetta or the choral prelude (if the time allows). Remember Bach's Orgelbuchlein?
Bonus Myth 1: You should always use classical tonal harmony in harmonizing hymns. No, on special occasions, you can surprise your congregation by harmonizing a hymn in chords containing seconds, fourths, fifths, tritones, and even modal harmony as well as jazz chords.
Bonus Myth 2: The style of hymn playing should always be chordal. No, you can make use of polyphony as well - make the parts more independent.
You don't have to believe these myths, if you want to make your hymn playing and church service playing in general much more creative, brave, and rewarding. Instead always ask yourself, "why am I doing this or that, why am I playing this hymn this way?" and try to challenge yourself with: "What if...?"
Sure, not everyone in your congregation is going to like the change in your playing but you are not trying to please everyone. It's not your job. Your job is to explore the boundaries, what works and what doesn't. Your job is to be an agent of change.
Let the people take part in your explorations, communicate with them why are you doing this. Then some of them will gratefully cheer you on and become true evangelists for your cause.
Don't be afraid to do something that matters to you. They didn't like Bach's hymn playing in Arnstadt after he came back from his study with Buxtehude after that Christmas of 1705 either...