I'm not sure if this would fit your needs, but you could try the
Despite it's name, it is also capable of spreading mobs, with the in Minecraft 1.8 newly introduced
/spreadplayers x z distance radius false @e[type=Creeper]
x z: The coordinates of where the mobs should go.
distance: The approximate distance the mobs should have.
radius: The radius of the area in which the mobs are spread (not a circle, but a square).
false: Just leave this at false, it is intended to respect teams when spreading players.
@e[type=Creeper]: The mob selector, the
type=Creeper is very important. You can write other mobs name in this, but do not leave it out, as that would spread itemframes, signs, minecarts... And that would be a mess.
Of course this only spreads the mobs, but you have to summon them first, but i think that is no problem, just use command blocks with
/summon Creeper x y z and you're good to go ;)