Interesting fact: If you rename a recruited hero, they can reappear at the base camp. Also, Protectomon are 'looted' during quests like items, and you can increase your chances of getting them by replaying storyline quests. So just replay one map with units with the 'loot on kill' ability. (Took me 18 times on the hydra quest to get them all.)
 "Protector IV.V"){kind=icon}
 "Immortal Souls: Dark Crusade"){kind=icon}