Feather picking is such a frustrating habit. Believe me- I could write a book on that particular subject. A few years back I had such a go-round with that business, I was ready to throw in the towel and quit birds altogether. So, let me share what I learned from my experiences with it. Chick Charm has the right of things, but I can expand on it a little more, I think.
Feather picking starts out either as a behavioral problem or a nutritional deficiency. No one can really say for sure which is the real culprit, but I believe that it starts out as behavioral in nature, but once the body realizes what a nutritional punch it gets from eating feathers, the habit gets positively reinforced. Eating feathers is like eating pure protein. The only way for birds to get a more nutrient rich source is for them to go after eggs. (Often you will see egg eating and feather picking going hand in hand.) Once the habit is reinforced enough, it becomes a very nasty behavior that is nearly impossible to break. Your birds are young and have only just started, so it is not too late to break them out of it. I think the combination of the terrible weather and getting coop crazy has made your birds destructive to each other.
The good news is that the weather has to break soon. (Not soon enough for me.) That may help a great deal in this situation because once they can wander around they are less likely to be in each other's faces and spaces. Once they can freely escape unwanted attention, they are less likely to be plucked at and unable to escape. In the meantime, if you can offer some diversions in their space that may help. Increased roosting spaces at different heights and spread out from each other, and small so that only 1 or 2 birds may occupy one roosting space can separate them and give the illusion of space. Obstacles in the coop that birds can run around and hide behind are also a good diversionary tactic. Toys like poultry blocks, a head of cabbage in a mesh bag held just out of their reach so they have to work to get it, and BOSS mixed in with the bedding so they have to search and pick for it. The last one is particularly helpful because it also addresses the nutritional need for added protein. Speaking of that, you should also be looking at increasing their protein intake by using BOSS, mealworms, bits of unsalted/unprocessed meats, cooked eggs, yogurt (if they will eat it) and canned fish (no added salt). This only has to be a short-term thing, so don't despair and think your birds are going to be eating better than you because they are not for the long-term. This protein boost will just get them through the last of winter until they can get out and rustle up some bugs and protein sources naturally.
Once the weather improves, keep an eye on the flock and see if you are still seeing the bad habits. If so, then more reeducation is needed. We can address that later on if needed.
The birds that have been damaged need to be Bluekoted and put back in with the flock unless they are terribly wounded. The Bluekote should hide the damage enough so they are not bothered. It is often better to remove the culprits from the flock rather than the victims. You would be surprised how the flock reacts when a bully is removed. If you feel it is warranted, there is a product on the market called Hot Pick or No Pick that can be sprayed on the damaged birds to make them taste nasty. It is slightly effective in the beginning, but needs to be reapplied often, so it can be an onerous task if you have several birds. There are also the very effective pinless peepers, but I wouldn't worry about that extreme, yet. They would be part of the reeducation I mentioned earlier.
Anyway, I hope this has been helpful. Sorry for the wall of text. Good luck.
