Well, I have seen my teeny tiny Pygmy Cory biting my Betta's tail while the Betta was at the top of the tank resting in a bed of moss. I don't think it did much damage, and it lived for ages with 30++ Ghost shrimp and they all lived together.
Are they Neon Green Kubotai / Microdevario kubotai (Microrasbora kubotai)? These guys are very busy active fish and although they mainly are at the top, I would be suspicious.
Not 100% here for which new fish. But that when there are new additions, there is a reduction of free space and a new volley for areas /territory, and the peacefulness which might have been enjoyed before when the dynamic changes, and these new fish, were not raised together with the other new fish.
Rabbit snail should be innocent!
As for where the shrimp bodies end up it would seem the flow / current of the tank will make a body go to corners. With the heads nipped off, sound like a quick bite off, of it and still stay up top, not going after the whole shrimp?????