BastyPutt
Yes, your Polish is a cockerel...
I guess, knowing the nature of MG, I am really confused on treating the whole flock if only a few are symptomatic.Due to the fact that MG is very contagious. It can be carried on your person, clothing, the shoes you wear, equipment, tires on a vehicle, feeders, waterers etc etc etc.
Contagious yes. It is safe to assume all the birds carry it. If they don't, then they will if OP has no intention of culling. But obviously you know treating the flock doesn't make it go away or any less contagious.
The only reason you would need to administer an antibiotic in this case, or any for that matter, is if a bird is in distress - which is why you would treat birds within the flock whenever they become symptomatic. That could be at the onset of a less serious foamy eye, or obvious signs of respiratory distress. With an MG flock it is going to be something you would learn when would be the right time for you and the birds.
But administering birds that aren't symptomatic will just make them more resistant to the antibiotics when they are symptomatic and actually need them.
If I'm missing something, please let me know!