I am dubious e-cat would kill the cichlids in this scenario. It does not put out a full strength zap when the danger is not mortal. It will simply zap them mildly, enough to let them know to leave it alone. And the cichlids will remember that. If not right away then eventually. I am talking in theory though, not from experience.
My e-cat grew slowly, from 4" to maybe 7" in 1.5 years. They can grow to 3 feet, however, this genus is full of cats, 19 total, I think. Not so long ago, they were all called Malapterus electricus. Not anymore. But they look so much alike that it is hard to tell many apart and there is no history with photos to help with that task either. I am sure not all of them will grow to 3 feet.
Here is a sample of a 3-footer!
http://www.planetcatfish.com/forum/viewtopic.php?f=8&t=28890&hilit=+electric