droopy wing/weight loss

You're doing what most responsible breeders do in culling if Marek's is suspected Bodger.  

Just make sure the biosecurty between your original birds and your new "girlie" ones is good  

Marek's is not transmittable through eggs as you know Bodger !

The culling policy of breeders as you say, is to remove susceptible lines and perpetuate the more resistant ones.

Some breeds are more susceptible like Silkies and Sebrights, I'm not sure what others.  And rather than being a disease it is a syndrome with about 4 distinct symptoms that the chickens may have one or more of.