I have never had a hen get gassy and die from too much greens. Now that the grass is growing again down here I put a hopper of grass clippings in the yard each day on top of the greens that we feed from the garden. They scratch through it, get what they want and leave the rest. As far as I have experienced chickens with a wide range of feed make appropriate choices about what to eat and what not to eat. Chickens who don't get greens might eat say tomato leaves because they are starved for greens, but mine would pass them up. I often test out new weeds by just offering them to the chickens. Some they gobble, some they ignore. Even some that I know are safe for them get ignored. I presume if they had no other greens they would eat them. I don't know how they know what is safe, but they do. I used to think that I had to research each weed to find out if it is OK, but now I mostly let them tell me.
My husband and I used to dumpster dive for the chickens. If we got a big haul of greens we would just chop it in chunks and put it out at once rather than have it rot. With ice berg lettuce they usually picked through a bit and left a lot. Greener lettuces and other greens they pretty much ate it all or all of the best parts. Never caused a problem. But I am glad to now produce my own greens for them as they are healthier.
Our chickens live long lives and that becomes a problem for us as we love to breed chickens and don't want to cull laying hens. So we always have a bit bigger flock than we really intend to have. My point is that in the 20+ years we have been doing this we have gotten better and have little problem with disease or other health problems. I may not have everything right or know everything but we must be doing things pretty well to have more birds that we even intended to have that are vigorous and live long and healthy lives.
I am just now excited to learn about fermenting feed. Always something new to learn. I knew about sprouting and just never ran across fermenting. First experiment seemed positive and I am glad to find useful information on the web and on the BYC thread about the topic. If it makes my hens even healthier I guess we will have to start selling hens again, because raising baby chicks with mothers is one of the most wonderful experiences in the world