I Think there was a sizeable pool to begin with, the early releases in that list is not listed by order of release but when there was a thread created about the character in the Character Discussion forum.
My educated guess on who was in the game at the start:
*, Black Widow, Modern, *, Hawkeye, Classic, *, Iron Man, Model 35, *, Venom, Dark Avengers Spider-Man, *, Storm, Modern, *, Juggernaut, Classic, *, Yelena Belova, Dark Avengers Widow, **, Spider-Man, Bag-Man, **, Storm, Classic, **, Wolverine, Astonishing X-Men, **, Magneto, Marvel NOW!, **, Bullseye, Dark Avengers Hawkeye, **, Daken, Dark Avengers Wolverine, **, Thor, Marvel NOW!, ***, Loki, Dark Reign,***, Ragnarok, Dark Avengers Thor, ***, Spider-Man, Classic, ***, Hood, The, Classic, ***, Magneto, Classic, ***, Doctor Doom, Classic, ***, Black Widow, Grey Suit, ****, Wolverine, X-Force,
This is mostly based on who was already in the game when I started playing in mid-November 2013 as well as an old announcement of a "Newcomers" Versus tournament that featured 2* Hawkeye, 2* Moonstone and 2* Captain America (and maybe a few more I don't remember off the top of my head), so I am reasonably sure they were later additions.
It's probably fair to assume that most characters that are featured in the prologue were playable (though maybe not complete) when the game was released.