Bone Heat Weight problems got in the way of what I wanted for Giru and Casey's necklaces before, but figured out a solution and got proper cuban chains on 'em. Right on time for a shorter animation soon.

Case someone else could use this info, first try scaling everything up by 100, and parenting with automatic weights. If that fails like what happened here, then go bigger. If THAT fails, while everything's still big, hit M to merge vertices and select by distance, raise the decimal in the popup to the highest amount possible that still has good geometry. Then I parented, succeeded, and then I shrunk the model.