I have had great experiences with my Bantam girls, less great with the roosters.
Cochin Bantams Sally, a Barred, and Gabby, a Red Frizzle, were raised with full-sized chickens and had no issues. I was concerned about how Gabby would deal with our harsh winters, but her previous owner pointed out that Gabs had done just fine through two bad weather winters, and she continues to have no issues.
No one sells that I'm aware of sells sexed Bantam chicks, and when I added three Bantams two years ago, I ended up with two roosters. All were supposed to be Cochins, one boy was not. The Booted Bantam boy was a total jerk who attacked the rooster he was growing up with, visitors and me. He went off to live in a rooster flock elsewhere and is doing fine.
Once Pauly was gone, BIlly Boy -- the former "victim" became a jerk, too. It took a long time and effort for him to realize that attacking me was not acceptable. He is very attentive to his girls, tries to keep them safe and lets them have first choice of treats. A light Blue Cochin, he's beautiful but I doubt we will ever be good friends.
The little dark Blue hen was wonderful. Like Sally and Gabby -- who free range with the others during the day but are housed separately at night -- she knew her own name and was an absolute doll. We had an extremely tough bout of frigid weather in December where all the chickens piled into the same nest box. She was on the bottom and was smothered.
Sally, BTW, has one bad foot and leg following surgery for a stricture. She hops with great gusto and is fierce -- she runs off any full-sized hen who messes with her. But, she had never shown any aggression toward people; she just stands her ground when someone tries to bully her -- or, in the case of Billy Boy, mate with her. She runs his fluffy feathered butt under one of the coops every time he makes an attempt.
My little jewels were raised with full-sized girls and do fine with bigger chickens. They were the only Bantams in a mixed flock at their previous home.
The girls' eggs are small, and they have -- like big Cochins -- a tendency to go broody. But I totally love Cochin Bantams and think they could be a great option for you. They are everything that's wonderful about chickens, wrapped up in a tiny, cute package.
Best wishes with whatever you decide!