We only have 1 Speckled Sussex, so I don't know if our experience is common or not. But I'll share what we've found.
She is the Alpha hen (they're just over a year old), but not mean. She just gets to treats first, etc.
When she was a couple months old, we thought she was mean. But we realized that she wasn't mean at all, she just doesn't like to be picked up. She'll run to us for treats, take from our hands, and is generally quite easy going. But she still doesn't really prefer to be held the way we can hold our others. I've had some other people tell me the same.
She's also a "dumpling" (that's her nickname) in that she's a rounder shape than our RIR or SLW (the other breeds we have are just chicks).
She lays a light brown egg, usually with white speckles on it. She's a good layer, but did slow down in the winter more than the RIR.
She has a beautiful green sheen on her brown speckled feathers.
Good luck with yours! I think you'll like them, they'll be a nice mix in your group.