In quarantine, there can be NO contact between them at all so they can catch nothing airborne. A good sized dog kennel or separate pen is good. Lice or mites are easily remedied during quarantine, but you'd have to upend him and take a quick look around his vent or under the wings for little...
I just posted about quarantine-look at the Sticky at the top of the Diseases & Cures. I put Hawkeye in a large wire dog kennel in my bsmt bathroom, dusted him and his bedding with DE, fed him proper food (he'd only been fed corn), yogurt, vitamins, handled him and watched him. A disease can take...
Just be sure to quarantine him away from the flock if you get him, at least four weeks. When you introduce him, the head hen may attack him, but he'll be able to handle it just fine. As far as yellow feathers around his neck, it's anyone's guess. That's pretty vague and could be a mixed breed even.
I added my rooster when he and my ten girls were about 11 months old. They really took to him. What is hard to get used to is the feathers missing where he grabs them behind the comb and on the lower back where he grips with his feet. Add moulting to that now and you have a few really sad...