The primary difference between layer feed and grower/finisher is the calcium content. Yes, it is best if you feed grower/finisher until they get close to laying. But you won't kill a pullet with layer feed if you switch her over to layer feed after she is old enough. You might damage a young bird's kidneys with an excess of calcium if you switch to layer feed too early.
Do you have a reference for your statement: "If you put them on layer feed before that you risk them laying too early or too large an egg and dying from it"? Because that statement is not consistent with anything I have read or heard, and it appears to be BYC style misinformation. I could be wrong. If you have a reference I would be very interested.
As for layer feed potentially killing non-laying birds, remember that there are few absolutes in this world. I have a rooster in with my laying flock. He has been eating the same layer feed as the hens for two years with no problems. When he first moved in with them I fed them all grower/finisher with supplemental calcium free choice for the hens. Got way too many soft eggs. Now they all get layer feed, including the rooster. Not ideal for the rooster but he is still healthy. Other birds might not be healthy in the same situation. Not all birds have the same requirements.
