On a legendary or rumor level, how do Horde and Brilliant Commander interact w the Start Area? Is that considered an "Area" and thus gets the benefit of the 2 extra monsters or upgraded red monster?

Or just the numbered Areas?

The Start Area is definitely an Area (nothing in the rules says it doesn't count as an Area like any other.) Each dungeon level from the deck is one Area (effectively combining any intrinsic properties of "the Start Area" with "Area 1") and these cards still trigger in regular dungeon levels. Otherwise they would be next to useless.

So I would say yes, they do trigger on the "Start Area" of a multi-area dungeon level whenever those crop up. Keep in mind, however, that Hordes requires at least one monster to be in the Start Area in order to place anything. You must place Hordes monsters adjacent to pre-existing monsters in the area just revealed - if there are no such pre-existing monsters, Hordes does nothing.

The rules don't specifically address this situation as far as I know, and I couldn't find anything in the FAQ, so barring any reason to think the Start Area is any different from any other Area rules-wise, that's the answer I would go with.