It's really your decision whether or not to cull the rest. If you want to get rid of it completely and not have to deal with it, that's the way to go. I have read here on BYC that you should cull the entire flock if you have mycoplasma. I don't agree.
A couple years ago, I had respiratory symptoms show up in my birds. I took 2 live birds to UCD for testing. Results were inconclusive but mycoplasma was suspected. (BTW, Walt recommended the Turlock lab, rather than Davis.) I then took 2 more birds to an avian vet in Roseville. She couldn't find what was wrong, either, said that she depends on the lab testing to determine what was wrong. They did not have worms. She said that in my case, since I have birds that are hard to replace, she did not recommend culling. She said any free ranging flock is going to get exposed to mycoplasma, and most are, so it is unavoidable unless you lock them up. She said that free ranging birds with access to sunshine & green grass are actually healthier in the long run, so it is worth the risk.
It does get spread through the egg, but if you cull birds with symptoms as part of a long range breeding plan, you develop resistance in your flock. So that's what I plan to do.
JMO.
I wish you the best,
Kim