Where did you get that information from ?
The speckled Sussex is not an aggressive breed!
https://countrysidenetwork.com/daily/poultry/chickens-101/speckled-sussex-chicken-favorite-backyard-breed/
you want speckled Sussex? Get one then, it doesn’t really matter what breed you get unless it’s a polish or silkie. You already have an aggressive breed (barred rock)
All birds have different personalities, you could have a real sweet barred rock (aggressive breed) and have a real mean buff...