Thanks, I had to read that a coupe of times but I think I've got it. You have experience with chickens, not a rookie. North of the equator so not the right time of year for a molt. The ones in question are mature, rooster and hens. Three months ago you added game bird conditioner to their diet. Two months ago you added the Australorp. One moth ago you started seeing the feather loss.
Let me think out loud about that Game Bird Conditioner, not sure why you added it. It should have about the same protein content percentage as your Layer feed, maybe marginally higher. It is low calcium and a little low fat, intended for that growth period of game birds from Starter until they are ready for their adult feed, not their laying period, just like Grower for adolescent chickens. How much of their total feed is this Game Bird Conditioner? Is it a significant portion or more like a treat, say 10% of overall feed? They don't forage so basically they eat what you give them. Brittle feathers is basically a nutritional defect. It's not that they don't eat the nutrients they need for feathers but more that their bodies don't process them right. The difference in fat is pretty small compared to Layer, but one trick to make feathers softer is to trickle oil on their feed, the oil conditions their feathers. I think it would be a real stretch to lay blame on that on that feed, the differences don't seem drastic enough. Many of us feed Grower to laying hens with oyster shell on the side.
But nothing else makes sense either. You may have changed flock dynamics some by adding the Australorp, but she was already laying so should act like an adult. They were fine for seven months with that set-up without the Australorp. In spring the hormones may get stronger in a rooster so he may become more vigorous, but in Florida that should have started sooner. Still, it may be a factor, why he is more vigorous in enforcing his dominance.
I should have asked earlier, what does the feather loss look like? I've assumed it is normal rooster wear, on the back where his claws rest or the back of the head where he grabs. Is it instead overall or other certain places. Have you checked them for mites and lice? Or is one of of the others plucking feathers?
You probably already know this but if all the feather came out that feather should grow back reasonably soon. If part of the feather shaft is left, they will not grow that feather back until they molt. So even if you solve the problem the feathers may not immediately grow back.
Nothing jumps out at me and says "this is the cause". If their skin is showing it could be cut by his claws or maybe spurs. It sounds like you'd be OK not hatching these hens' eggs so i'd probably move them out and see what happens.
Good luck with it.