I removed that requirement, so no need of the research.
About the ability- The ghouls cannibalize, but only their friends, when I put beside them corpses of enemies they won't cannibalize them.
Yes, the unit group is correct, I know that because it worked at first (on enemy footman corpses) but after a few runs of the map I couldn't order them to cannibalize (on enemy high elves, the villagers).
For your information, I'm ordering them to do it through a trigger because they are controlled by a computer player.
EDIT: It gets weirder. Although I took a part of their life away with triggers, they cannibalize only once attacked.