Hi,
You will need to use "C:\Program Files (x86)\Common Files\Adobe\XMP\Metadata Extensions" location.
For e.g. consider that you want to use the "Sample Extension" given with the XMP FileInfo UI SDK (available at Adobe - XMP Developer Center )
Then you will need to copy the Sample Extension folder at location "C:\Program Files (x86)\Common Files\Adobe\XMP\Metadata Extensions\".
And after relaunch the Bridge will show the FileInfo dailog with the sample view provided with "Sample Extension".
Follow the same folder and file conventions as Sample Extension.
Please refer the " Adobe XMP Metadata UI SDK Overview.pdf" that comes with FileInfo SDK for more details.
And regarding the second point, Metadata Panel and File Info are handled differently, so for metadata panel changes you will need to make changes via Preference in Bridge as it is handled by application itself.