Hello,
My team and i want to use physics in our babylonjs scenes but we are having issues setting a custom imposter. A - Which Physics engine should we use?
B - How can we assignee a custom imposter to a mesh and can the mesh be its own imposter?
Note: we want to use physics to drop a necklace around a human 3d model and for other future uses.
Hello and welcome to the Babylon community! For Physics we highly recommend using our newest V2 architecture and Havok physics which are a new feature of Babylon.js 6.0 
You can see the documentation here: Physics | Babylon.js Documentation (babylonjs.com)
And yes, you can have the mesh itself be its collision shape, all the available shapes are detailed here: Physics V2 Shapes | Babylon.js Documentation (babylonjs.com)
Thank you so much, cant wait to see what we can put together and do using physics in our scenes, much appreciated.