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Look up Chronic Respitory Disease, Mycoplasmosis, Air Sac Disease and Infectious Bronchitis and see if any of these illnesses describe what your birds are experiencing. The Gallimycin is probably your best bet at this point. A couple of these illnesses can be brought on by stress and since your birds where shipped it may have triggered something. I hope they are feeling better soon.
The 3 boys that I grew out that all had white in the hackles and fluff at the tail break, I kept one and put him in the laying flock, he has wry tail and if it weren't for that I would be test mating him for breeding even with the white in the hackle that seems to be un-noticeable now at 6 mos old, but if I lift his hackle feathers, the white is still there. He will only be staying until my youngest Splash Copper roo is old enough to take his place. I noticed the white in mine about the same age you did with yours, it disappeared on the waaaaayyyy overly red boy by 6 mos, but the other 2 it didn't. They were well built, large sized birds and the 2 we put in the freezer dressed out very nice.
Give them some more time to see how they mature before tossing them out.
I would say yes they have CRD, from a mycoplasmosis. I have been giving tylan in water for the last week. They had been on the gallimycin for 2 wks previous without a getting completely better. Now they are worse! I guess I will go back to the gallimycin, but how long should I keep that up before I give up? Have you ever "cured" CRD? I really do not want to give this to the rest of my flock... but I can't keep these 4 in quarantine for ever.
The feathering is exactly how you describe the 2 roosters you had. I do not think they are worth keeping for breeding.
p.s. Math I would put them outside in a separate pen but it is too cold for the next week or so.