In general, one bite won't kill them. Dosage is important. And I agree with Trish, if they have an option they avoid things that can harm them. But a key is that they need an option. If they have a wide variety of things when they forage I don't worry about it. The risk comes in when they live in a barren run with no green stuff and you offer them something. Even if it is harmful and they would generally avoid it, if it is the only green they can get they just might eat it in quantity.
An example is tomato plants. The fruit is fine, mine love ripe tomatoes. But the leaves contain a substance that can harm them. If they are foraging and bite a tomato plant it's really bitter so they leave it alone and eat something else. But if they are living in a barren run and you toss them tomato plants, they might eat the leaves, maybe enough to harm themselves.
Sumi, one of your questions is what plants do I avoid in their foraging area. My answer is none because they have good foraging options. That's the easy one.
Your other question what should you never offer your flock. This is mostly for those that don't have good foraging. I avoid avocado in any case, I'm not sure they have a natural defense against avocado and it is dangerous. Avoid moldy foods. Most mold won't hurt them but there are a few types that will and I don't know the difference. Moldy feed is especially dangerous. I'd avoid tossing them tomato, pepper, eggplant and potato plants if they have limited forage.
Avoid green potato skins and green potatoes. Regular white potatoes and potato skins are fine but the green ones contain a dangerous toxin. You should not eat green potatoes or potato skins either. Even cooked they are still dangerous.
Uncooked dried beans contain a toxin. Cooked beans are fine. it doesn't bother me if they eat a few but don't dump a pile of uncooked dried beans where they can get to them.
Apple seeds and other fruit seeds are often looked upon in horror on this forum. They do contain a toxin but the dosage is so small it's not going to harm them unless they really eat a lot. Mine forage in an orchard and eat fallen fruit and seeds all the time. No problems. By the time they have eaten enough of the fruit to get to the seeds they are not going to eat enough seeds to harm themselves. But when I make jelly or jam and have a pile of seeds, I dispose of those where they can't get to the pile.
Other than moldy foods or avocados, the key to me is moderation. A bite of green potato skins, uncooked dried beans, or fruit seeds won't bother them, but in quantity they can. Even great treats like cabbage contain a substance that can harm them, but you would have to feed them large amounts daily for several days or maybe even weeks for them to build up enough toxins to harm themselves.
