Best keep the little guy in a seperate dog crate or sectioned-off run for a week or two so the flock and young cockerel have a chance to get to know each other safely. After that, you can feel free to let the small roo out and-basically-have at it! But I wouldn't just leave him be at the end of the day, just in case you have bigger problems and need to take him away again. If he draws blood you have to patch him up and keep him away until he heals before trying again. Sometimes it takes more than one try.
You should always expect fighting anyway, from the adult rooster, as well as the hens. I wouldn't be surprised if the hens gave the little guy more trouble than the other male does. Hens get testy when there's fresh meat that wants to dominate them.
Higher ranking hens won't have trouble putting the small-fry in his place...for now. They'll all have to re-adjust again so after the intro, it might take a few weeks for things to get back on track again.
Make sure there are at least 8 hens for EACH rooster to share (with two boys, that's obviously a minimum of 16 hens or so). Any less and you might have a problem. Both are going to want girls of their own and if they don't have enough to get that 'itch' there's going to be more than just a few weeks of fighting. Take it from me.