Quote: Glad to hear she is doing well!
If you have MG on your property and add new chickens, they will be exposed. There is a possibility they could become ill (show outward signs of illness). Not all chickens exposed to MG will show symptoms, but being exposed (living with chickens that have MG), they will be considered carriers.
The only way to know for sure which illness your chicken(s) have/have been exposed to would be through testing and/or necropsy.
If you are dealing with something like Infectious Bronchitis (IB) then recovery is fairly quick (up to a month), they will be "carriers" for anywhere from 5-12months. It is not "long lived" on premises (about 4wks), but can be "shed" for a while longer. Proper ventilation is helpful with IB and cleaning/disinfecting the coop would be a good idea as well. IB can have long term effects in egg layers - drop in egg production, soft shell, egg quality, etc.
Almost all respiratory illnesses can impact egg production/egg quality and can possibly cause problems with internal laying/reproductive disorders down the road.
As to whether you can "safely" add new chickens to your flock, this is a delimna we all face at some point in time. If we have "healthy flocks" there is the potential of introducing an illness with new birds or vice versa. You will have to determine whether it is relatively "safe" to add new birds, follow quarantine and biosecurity. Monitor your flock over winter and take steps to treat any illness you encounter or have a necropsy or testing performed on any that develop symptoms.
https://www.backyardchickens.com/a/...often-underestimated-part-of-raising-chickens
http://www.thefancychick.com/PoultryBiosecurity.html
https://www.backyardchickens.com/a/adding-to-your-flock
http://www.the-chicken-chick.com/2016/08/biosecurity-for-backyard-chickens.html
http://www.the-chicken-chick.com/2012/12/quarantine-of-backyard-chickens-why-and.html