That looks like it might be a hernia to me. That large a protrusion is not just inflammation and is due to something being amiss internally. The red inflamed skin will be due to poop building up and burning the skin and also her repeatedly preening at it either to try to clean it or because it was itchy or burning.
Unfortunately she is going to be prone to getting a messy butt because that protrusion means her poop cannot fall free like it normally would. Keeping her skin greased up with an antiseptic/antibiotic ointment will help prevent future poop sticking and irritating the skin and also help the skin to calm it down, but the hernia (if I am correct) is a problem and will eventually cause her serious health issues if it isn't already.
Has she been laying recently?
The other concern is that the cause of the lump is salpingitis (infection of the oviduct) but usually that is a more general swelling around the vent and above it as the oviduct is above the gut, rather than a quite defined protrusion like this below. Salpingitis is usually fatal but can take several weeks or even months to get to that stage and probably a lot of discomfort for the bird.
I would give consideration to your hens diet. It may not be relevant but it can be.... Too many treats, in particular carbohydrates like scratch, bread, pasta etc can cause a build up of fatty deposits in the abdominal area and some cause a constriction of the gut and/or oviduct causing the bird to strain excessively and a weak spot in the muscle can then allow the gut to pop through.
If it is a hernia, it might be possible to correct it with surgery but I imagine that is going to be very expensive and risky as anaesthetising chickens is difficult to get right, so there is a higher chance of fatality from it than most other animals, I believe.
I'm sorry I am not painting a very optimistic picture here and certainly not a straight forward case of treating mites in my opinion.
The fact that in this case all the Australorp hens are missing feathers and all the red sex link hens aren't, makes me doubt that it's a coincidence.
In my hen house Isa browns are more energetic than my australorps and they know how to be mean but the big boss is my two and 1/2 years old australorp rooster. In fact all my australorp hens are quite shy and I noticed that they prefer to spend a lot of time together instead that with the Isas and when an Isa tries to attack an Australorp then the roo immediately protects her.
I love when roosters try to protect the hens when in danger especially when it comes to chickens trying to hurt other chickens. I have a little black copper maran cockerel who is 8 weeks and the rest of them our pullets, I sure hope he protects the hens like your rooster does. I don't think the chicks are ready to be with the big ones though because I took them in the chicken area today outside and my barred rock and Easter egger were chasing the pullets all around the pen and my Delaware picked up my little rooster by his neck feathers and started throwing him in the air and kept grabbing him and running off with him so I moved the chicks back to their little area with their hear lamp since its so cold outside. I might wait another couple weeks.
