At the Barracks, select "View Soldiers", select a soldier, then select "Customize". Note: This can also be done from the "Customize" menu at the "Deploy" screen. Unlock the following characters by changing the name of the soldier to the corresponding names. Note: This will prevent achievements from being earned.
During a Terror mission that has evil Crysalid terrorists, killing the Zombie Civilians isn't required to finish the mission. As long as all Alien forces have been defeated the mission will end successfully.