What are the reasons you want to add a rooster? Do you want to breed them? Looking for protection for the hens? If you don't really have a reason, I would skip the rooster because they do end up being more work. If they overmate the hens, you will have to purchase hen saddles to protect them. If his crow bothers the neighbors, you'll have to buy a no-crow collar...etc.
BUT if you really want a rooster, I say go for it
I don't think it will affect the hen's personallities at all and if you are able to find an adult serama rooster that would be even better because you would already know his personallity. If you can't do that, here's what I did when I had a rooster chick.
He developed faster than most cockerels and at 6 weeks he had a really big comb and was already attempting to crow. He was also very aggressive. He pecked me whenever I reached into the brooder, wouldn't let me handle his hatch mate (who was a pullet) and was all around a nasty little guy. So what I did is I would reach into the brooder with heavy gloves on so even if he pecked me it didn't hurt. Then if he pecked me or prevented me from handling Penny, I would pick him up and carry him around. I did not allow him to get out of my grip. This showed him I was boss. Taking him away from his little mate made him uncomfortable and he didn't feel as dominant, meaning he turned into a nice little chick who would allow me to stroke him and play with him. I did this every single time he showed bad behavior. He turned into a nice guy within 2 weeks and showed less signs of being so nasty. Unfortunatley he & Penny were attacked by a raccoon inside my garage and didn't make it. I miss those little guys all the time even though he wasn't very nice lol.
