Currently the guidelines are set more/less like this:

• friendship_is_magic - Only canon things/characters that are directly associated with Friendship is Magic get the tag. Rarely needs to be added manually since there are so many character implications already in place.
• my_little_pony - Friendship is Magic, Equestria Girls, MLP-styled fan_characters, older generation ponies. Basically most everything pony/brony related ends up with this tag.

The split of friendship_is_magic/my_little_pony was partially intended to separate canon from non-canon, and partially to separate new pony from old pony.

As long as a post is reasonably MLP-related it's fine for it to get the tag. Since my_little_pony is one of the more commonly blacklisted tags, we generally prefer to keep it overtagged rather than leave it undertagged (and run the risk of the blacklist not working as expected).

There are also stylistic cues that, while they aren't exclusive to MLP, they do at least convey some association to MLP. For instance:

• Thick, stubby, soft, often featureless legs and hooves, usually lack feathering or fetlocks (though some characters still have it).
• Smooth body (e.g., lack of fur tufts)
• Cutie mark
• A more pronounced, squarish jaw in male characters
• A more bird-like head in female characters. (g4) (g1-g3 have more horse-like features here)
• Eye with a large, distinctive colored pattern around the iris (g4) or symbol in the sclera (g3)
• A simple curved line used to mark the inner ear

Beanjam said:
Frankly, I think you deserve a red mark for shit stirring. You know damn well that you're supposed to put that sort of thing in the reason field of the tagging form.

The reason field is barely looked at by most people unless there is a conflict going on. As long as the discussion stays civil there's no reason that the comments can't be used for that purpose as well.

Though, if it's related to changing/building guidelines for a particular tag, it's better to bring it up with the forums instead, as it's a lot easier to keep track of discussion on the forums than the comments section of some random post.