I'm kinda new to this forum, and new to chickens too, but here's my experience with introducing a roo.
We started with 7 hens that were a few months old. We had them for a few months when a fellow chicken lover asked us to take a "really sweet roo". He was about a month younger than the girls, and smaller because of that. He wasn't so much sweet as he was shy and/or scared of my 7 girls.
You will read on these forums all the good reasons that you should separate any newly introduced chickens for 3 weeks or so, and how to best accomplish that, but we trusted the woman who gave us the roo, so we did not. We simply put him in with the girls and watched carefully for the first few days. Our situation is different than yours because he was smaller than them, so we were not worried that he would hurt them. In fact, they bullied him and chased him around constantly. And then he grew bigger than them! All of a sudden he was not so sweet, even to the point of trying to attack me whenever I went in the coop to feed them. Someone on here told me that once the hormones kick in, that everything changes, and it did. At 3YO, your roo should not have that issue either.
All this to say that even though I think he is too rough with my girls now that he is so big, they don't seem to mind him at all. I think my Big Mama is still at the top of the pecking order, but he now fits in with them well and they all free range together in a pack (or is that a flock?). We got him in July, and you would never know that he was not part of their little hen society from the beginning.
As for protecting them from predators though, he has been no help at all. We are in a very rural area, so there are all kinds of predators to watch for, but since getting him we have lost one hen who just disappeared, had one who was attacked by something and recovered, and another who was pecked by her fellow hens, and the rooster was not their protector in any of these cases. I know that they all stick together, so I am sure that he was there to witness these events, but he either did not know what to do, or was just as scared himself.
If I were you, I would take the roo, because they add so much personality to the group, but I think there's no guarantee that he will protect your girls. I am afraid to let mine continue to free range, but am just starting to get used to the fact that we are going to lose hens to predators and that is just a fact of life around here.