From interviews you might get the impression Kieron Gillen wanted to create much larger cast, but decided he wouldn't be able to tell a good story with it and cut it to bare minimum. He plans on slowly expanding it as the series goes on. He also wanted to use Eli, but was told another writer already called dibs on him.
Word of Dante: It isn't quite clear if Wiccan and Speed are actually mutants. Just about every fan assumes so, which probably led to a few times that they've been referred to as mutants in the series. Wiccan's powers did activate when he was a young teen, and in the X-Men Bat Family CrossoverBattle Of The Atom, Wiccan is a part of the X-Men, so there's that.