I agree, the rooster should be bantam, pick one that you like! People always have roosters to give away, and if he’s older you can ask about personality and find a gentleman. Be sure to check him over for mites and other diseases or injuries, you don’t want to bring a sick bird into the flock. If the place he’s coming from is clean (you might want to pick him up at the place where you get him so that you can see) and nice then he is probably okay, but always check! When I integrated my new rooster I put him in a cage in the coop at night and let him free during the day. I have a large pen, so I set up a little hidden spot with food and water for him because the three older hens didn’t appreciate his presence. Within four or five days everything was perfect, and now he’s an accepted member of the flock—although maybe not in charge of the older ladies. But he does enjoy bossing around the young ducks and pullets! When you get him make sure to check his crop at night to be sure they are letting him get his share of food. I would also encourage trying to find one around the same age or older as the hens—a very young cockerel won’t bode well with the mature ladies, and may scare or over-mate them. An older rooster will usually be a gentleman and has some experience with hens.
I love having a rooster around, it completes a flock in my opinion, and I enjoy watching them all interact. Good luck!