^I haven't had this experience with chromis, with urenoma or with their killing each other off, and this has included thousands of chromis over the years.
The urenoma is often a matter of how they are sourced, find a good LFS where you can observe them.
As far as killing each other off, I have found multiple feedings a day really cut down on their aggression toward one another. Even when some start spawning and aggression increases they still don't kill each other if healthy.