Sorry to hear that you have myco in your flock.
Medicated chick crumbs are for the prevention of coccidiosis, which is an intestinal infection by a parasite, and is often fatal to chicks under 8 weeks in less than 24 hours. After eight weeks, they have normally built up a bit of resistance so that a cocci infection doesn't bother them so much.
How old are your chicks? You will need to keep them completely quarantined from your flock, and ideally they should never come into contact with the adults - ever. Once you have myco in your flock it is there forever. The mycoplasma infection itself isn't a very bad infection, but it does make the hens extremely susceptible to secondary bacterial infections (which is where the snotty beaks come in). If the hens recover from one respiratory infection, the mycoplasma infection remains latent until the bird is stressed (gets hot, gets cold, moves house, is botheres by a predator etc...) and then it flares up, leading to another round of respiratory infections etc.
The old fashioned way of treating this was to cull the entire flock, rest the land, and then restock. If you want to go down this route, I would suggest you rest the land where the current affected adults are for at least 3 months, and be generous in your applications of Stalosan F for the ground, and virkon for the coops.
In my personal opinion (and it is just my opinion), although heartbreaking, it is better to cull the flock and restock, because if you have myco, you should really maintain a closed flock, otherwise you will just infect any new birds introduced. With backyard flocks, where hens are more pets than livestock, emotions come into it far more than if they are commercial flocks... and it is one of the reasons for a huge increase in myco cases acorss the country.