And surprisingly the only thing I found left of that chicken was a couple bones. Clean
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My first thought is that you have a weasel. There are different members of the weasel family, some larger than others. Typically weasels go for the head, kill several chickens, and can squeeze through some pretty small holes. The smaller ones can be extremely hard to keep out of your coop. You need to seal all holes with small mesh wire, cracks around doors can often be big enough to let a weasel in. I don't know what area or even country you are in so I don't know what members of the weasel family may be native to your area.
You free range and I think you are talking about adult chickens, not immature ones. When you have a lot of room and you are dealing with adults it is typically not hard to integrate chickens. The problem typically come in when space is tight or you mix ages/maturity levels, which describes a majority of the integrations you read about on here.
I don't know what your coop looks like, size or anything else, or what you can do about the predator. Your integration issues may center around the coop, especially if it is small. Another possibility is that the new ones may go walkabout or roost in trees if you just turn them loose on your property instead of confining them for a few days so they see it as home. You can have issues but with adults and lots of room integration should not be that bad.
Good luck!