That is what I'd do unfortunately. MG is highly contagious and from what I've read it is practically impossible for the other birds not to have been infected, symptoms or not. Especially considering where they came from, and the other possible diseases they could also have. If you keep them they'll basically need to be quarantined for life, and any new birds you get will be infected too. This is why respiratory diseases are such a major issue, and why your necropsy was so important. It would be terribly difficult to cull them just on a hunch, but now you know what you're dealing with, and the options are very limited. Even wild birds exposed to your flock will become infected and spread it.
Do research on how long MG can live without a host. I think it was only a couple weeks, but I would wait a couple months. Some say three months is best just to be cautious.
