Regarding Ridley, here are a few things I've noticed...
1. If you look at how the shadow moves, it would be Ridley moving on the same axis as a normal fighter, left to right in the playable area. With Yellow Devil in mind, this could mean nothing. But it's something to keep in mind; He doesn't come up from the background, like Yellow Devil does, and he moves across the stage in a fluid motion, unlike Yellow Devil.
2. This is more a play on semantics, and sounds like a Sakurai move. But what if, by "Other boss characters may appear," he didn't necessarily mean stage bosses? Bowser, Ganondorf, and King Dedede are all "boss characters" in their respective franchises, and since Ridley is a "boss" in Metroid, this may be what he was referring to. And since Ridley is on Pyrosphere, yes, there is a "boss" on the stage. And the caption reads, "Other boss appearances," which doesn't restrict the definition to being a stage hazard. The whole thing could just be a giant play on words. Also, it sort of begs the question why he specifies "boss characters" rather than just "bosses," or "boss stage hazards," but that might be able to be attributed to the translation
3. Going off of that, stages in Smash Bros. tend to be unique in their own way. Wily's Castle gimmick appears to be Yellow Devil himself, and the idea of the stage is to "beat Yellow Devil" and get the bonus explosion. If multiple stages were to have similar boss battles, Wily's Castle would lose a lot of its uniqueness, which doesn't strike me as very likely. Also, I don't think Ridley would explode, so killing him would do basically nothing, meaning he probably isn't a "stage boss," but he might be a "stage hazard." However, refer to point 2 above and point 4 below as to why I think this isn't the case.
4. It makes absolutely no sense from a gameplay perspective how a stage background would pick a character up (Pikachu). The player would be punished and unable to play for a short duration, which is horrible gameplay design. Say what you will about tripping, but it didn't actively prevent a player from playing for 5 seconds. This would basically be free hits, which makes for a horrible stage hazard in and of itself.
Just an interesting new take on the information presented. I can't verify it so I don't know if point 2 still holds in its original Japanese context, but it's still an interesting thought. Also, quick question, but did anybody catch Cranky Kong's silhouette in the DK stage shack? I didn't happen to notice it, so it might be an indicator of a certain "old kong" appearing as a fighter or assist trophy, as well.
