The Gematsu leak has been 100% so far, and it's been going for nearly a year and a half. It got WFT, and looks like it's right on Chorus Men too. None of its speculation has been wrong. It's legitimate, period.
I'm still a little skeptical, mainly because it claims we'll have characters as downloadable content. This doesn't mean I doubt the character guesses, though. I agree, getting 8 right, including multiple obscurities (wii fit, mii fighters) probably indicates inside information. I expected Chrom regardless, and at least acknowledged Shulk's chances, but now I expect them, as well as Chorus Men, who I hadn't even considered previously.
But just because all the stuff it guessed was right, it still doesn't mean it's 100% correct. People speculate that the info is a little dated, and that's why Rosalina wasn't predicted. If that's the case, Having info on things like DLC is a little odd, which usually isn't planned super early. Plus, it doesn't seem to me that Nintendo is willing to do dlc characters for smash bros.
I am hopeful, because that would be awesome, and the leak has been credible enough so far. However, regardless of whether or not it turns out to be 100% correct, in absence of absolute proof, I'll remain always a skeptic.