I think the primary issue was that the bantam silkies were in with standard type birds. Crested banties and standards probably shouldn't be housed together unless there is ALOT of space to get away.
Silkies can't always see that well due to crests and so can't see danger as well. With a crest, they won't be able to see if they got in the way of a standard and would be attacked for overstepping their lines without even being able to see that they over stepped boundaries.
If they were moulting, they may get a wilted comb, lose a bit of weight, and may slow down a bit, but should bounce back quick. I suspect that your standards may have been stoping the silkies from eating thus making them weak.
If the flock went after a silkie first, they could have now developed a pecking issue and took advantage of your standard hen who was weak for some reason.
Another thing, often people say silkies are doscile calm birds... I find this VERY true with heavily crested and beareded silkies. It's because they can't see! I have some with small crests and can see just like any other chicken and as a result, they act just like any other chicken. They move when you almost step on them, and do more normal chicken things, other than just standing there pecking at the dish trusting there is food to be eaten. They can actually get bugs and aim right too.
The sexlinks will probably be just fine with other standards and other sexlinks. If you want to expand for egg production, go ahead and get more standards. Of course, with any introduction of new birds, there will be squabbles, but do let them figure it out. It will take a few weeks. With your current coop, the "rule of thumb" would max you out at 8 hens. (4 sq ft inside 10 outside each) However some say you need at least 2x that space per bird, while others do fine with less.
I personally keep all my silkies in their own coop, and all the standards in a different coop. They do range together and are just fine, but have an acre or so of cleared land to roam, and much more of woods that they don't enter.
Perhaps if they are bored, you could give them something to do. Like hang food from the ceiling, or giving them some straw to dig though after sprinkling in some bird seed or scratch.
If they are eating feathers and picking due to lack of protein, which could be lacking if they are moulting and need the protein, supplementing with eggs, cat food in moderation, or finding a flock raiser or layer supplement with a higher percentage protein may help.
Good luck!
ETA:
If a moult was to happen, it is not uncommon to wake up one morning, and have the coop look like someone had a pillow fight. I have hen who is curently molting, and she's a heavy moulter. She's dropped about 60% of her feathers in two days and as she walks, more falls off. She jumped to her roost spot and left a cloud of feathers behind her. She is nearly bald with pin feathers just starting to show. However, she's not lethargic and does regular chicken things.
