It is kinda funny that Chrom ended up the odd man out here and as a result this... kinda breaks the Gematsu leak, which from what I'm guessing is probably an insider who ended up getting relieved of this position over at Bandai Namco VERY early on in the game's development and that his information is outdated.
That does seem like the case. For Greninja the leaker only said a Pokemon from X/Y, which as we found out at the E3 roundtable, Sakurai picked Greninja because he liked the characters design when he was shown the early X/Y designs, and that Geninja didn't even have an official name yet. The leaker also said recently around E3 that there was debate about if Lucas should be cut or not. Something like that would have been happening much earlier in development when they were deciding what resources would be spread out, not a few months ago when the game is in the final stages of development.
As we know from Brawl, they plan all the characters at the beginning and then make cuts from there. I wouldn't be surprised of Chrom was planned early on, but then Sakurai realized what many of us did that 3 blue haired Fire Emblem sword using lords taking up the character select screen is too much. So they cut Chrom in favor of using Robin who actually represented the player in Fire Emblem, and allows for the use of magic attacks, making them a much needed unique Fire Emblem character. They probably took the Chrom model they worked and decided to use it for Robins Final Smash since why let a character they worked on go to complete waste.
So yeah, Chrom is probably the special case where he was originally going to be playable but got changed. For Shulk and Chorus Men, I don't see them getting cut since they represent new franchises for Smash Bros with completely unique movesets. Plus as we saw from the Smash Run demo, there's a Rhythm Heaven ghost enemy in it which means a Rhythm Heaven character is playable. Of course Sakurai might have the Chorus Men in favor of a different Rhythm Heaven character, but either way, we're getting some kind of crazy Rhythm Heaven character that I expect to be insanely awesome.