I have read that it is best not to introduce a young cockerel too soon .... before he is ready to establish himself as head of the flock .... because if he is bullied he will possibly never have confidence to establish himself in his rightful position.

Not sure if this is true or not as most of our cockerels are hatched here and not bullied as such ..... kept separately until older.
Think really they should be in isolation for at least 3 weeks ..... know it's not always possible though.
I would let them all sleep together and if they are free range over a big enough area they will be able to escape the bully in the day and you shouldn't have real injuries. Do they have to go in the run first thing or can they go straight out to free range? As you know there will be a bit of "commotion" at first but if they aren't trapped in the run they should be okay. I would make sure there are a couple of feeders/drinkers to reduce stress on the new birds.