Letting a cockerel grow up with pullets can work sometimes, and other times it does not work so well. You have to figure that one out by watching as it happens.
Letting the hens grow up first and then adding a rooster is definitely one option that can work well.
As regards warning about predators, some hens will do that too. You do not always need a rooster for that. As regards protecting the flock, some roosters will and some will not. The ones that do protect will sometimes get killed. A good fence is far better than a rooster for protecting the flock.
For siring chicks, yes you really do need a rooster, not any kind of substitute

But in that case, since he will be the father of every chick you raise, you definitely want to think about the traits you want in the chicks, and make sure the rooster has the right traits. Egg color, laying ability, size, growth rate, temperament, and appearance are details that people often care about, although I don't know which ones will matter to you personally.
If you hatch chicks, about half of them will be males (Murphy's Law says more than half), so you need plans for dealing with quite a few extra males.