So glad the vet is coming. Fingers crossed they are helpful. I assume you are going to ask their advice about when to mix the new ones in. Please share what they say. I have some thoughts but I want to hold onto them until the vet weighs in.
I will try to get clarity.
I believe that the same doc that initially I spoke with for this is on this housecalls trip, "Dr Jill". On the phone call she said two confusing things, or I didn't hear her correctly. Complicating this conversation was the little ones having had a sniffles episode too right after arriving, maybe viral, maybe came with them, who knows. A person here claiming the be a vet if you go by their user name on BYC on my Diseases thread offered that me treating them with Tiagard wasn't a bad idea even if it is viral, to head off a bacterial secondary infection which can often happen. I believe I told the history of the littles to Dr. Jill.
Sorry to go long, but it's background for my situation, what she said here and what she'll say later.
She said she would not blend these two groups (now or ever? Not clear), and then said she never adds chickens until they are 18 months old. I'm sure I heard months. She says it takes them maturing and well into laying for symptoms of mycoplasma to develop if they have it. She integrates adult birds only.
By this idea, Queenie could have been carrying it, if this is what Hazel has, and not showing any symptoms. She didn't lay until well after integration, a few months later I think. Besides the possibility that the gang picked it up from wild birds. I'd have to look at the timeline of Hazel's first episode.