While with adequate space food and waer you can decrease the amount of cannibalism, it definitly has still happened to me on many occasions . When you keep similar sizes around each other, you can decrease this a bit. As Julie said, it is also a huge hassle getting them out. If you are getting leopard geckos, definitly get a few hundred mealworms and start breeding them. Many leo breeders feed exclusively mealworms and have thriving collections. Plenty of info on this forum about how to do it too.