Unless Legendary acquires the rights to the other Toho monsters, we will never see them in the Legendary Pictures lineup.Each individual Toho monster has its own licenses that Legendary must purchase in order to use them in their films, and I don't see that happeneing.
And no, Mechagodzilla will not be in the 2014 movie. And again, unless the rights to the mechanical Godzilal duplicate are purchased, it will not be in the sequel either.
Honestly speaking, I don't want it to happen. Legendary is a powerhouse in the movie industry today, and if they can't come up with their own unique monsters and stories to throw against Godzilla, then they don't deserve to make another movie.
Toho monsters are near to all of our hearts, but they have no place in this new American Franchise. They're Toho monsters, not Legendary Pictures monsters. This new Godzilla is a product of Legendary Pictures, and it should stay taht way.