In my experience, I think the Aust. ...once mature.. would be the one picking on your existing girls. JM2CWI don't mind waiting if needed, I'm just more concerned about their temperment and being able to not get overly bullied by my exsisting reds.
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In my experience, I think the Aust. ...once mature.. would be the one picking on your existing girls. JM2CWI don't mind waiting if needed, I'm just more concerned about their temperment and being able to not get overly bullied by my exsisting reds.

From my experience, Australorps would be better standing up to Reds. Sussex are known to be more docile. I've only had one of each, but my reading on BYC has tended to reinforce this belief. I had a RIR that was a bully, but the Aussie was #2 in the pecking order. They are both gone now. My sussex is only #1 now because she's the oldest. But she's a gentle leader, never bullies anyone.I'm just more concerned about their temperment and being able to not get overly bullied by my exsisting reds.
Well, depending on your luck, those birds would be considered getting elderly. Some people do have very long lived birds, they tend not to be hatchery birds. So that could affect your flock. In my own flock of hatchery birds, I would not be surprised to start loosing birds to natural causes at this age.
My point, is, that some Reds are a bit aggressive. Especially birds that have been a close flock in their own territory. I think the BA's are middle of the road aggressive, so they should be able to hold their own with the Reds, and as the Reds die off, they would be good to mix with Sussex next year.