Each chicken is an individual with its own individual personality and traits. Some breeds do have tendencies but within each breed there is a wide variety of how they act.
I've had Black Australorps together with Speckeld Sussex, Delaware, and Buff Orpington. Some Australorp were pretty high in the pecking order, some were lower. They were just as likely to do the pecking as the ones being pecked.
Buff Orpington are supposed to have a tendency of being mild-mannered and picked on. My top hen in the pecking order was a Buff Orp. That was against breed tendency.
I've never heard that Black Australorp had the tendency of being picked on. It is very possibe that someone you know had one that was at the bottom of the pecking order in the flock. That's an individual personality thing.
It is also possible, maybe even likely, that you may initially see one being picked on if you add one. Chickens can be territorial and often protect their territory from newcomers. And if you add a young one, the more mature chickens will rank higher in the pecking order because of maturity and are not shy about enforcing those rights. But this has nothing to do with breed. It is just normal integration issues. You are likely to see the same thing regardless of breed if you add a new chicken.
RIR have a reputation of being mean and bossy. Barred Rock are also sometimes reported like that. Yet you also see a lot of posts on here where their individual RIR and Barred Rock are not mean and bossy at all. It depends on the individual personality of the specific chickens you have. I have never personally had RIR or Barred Rock, but I would not hesitate to get some and mix them with other breeds if I wanted these breeds.
Good luck however you decide.
