Allow global terms across all blogs on a multisite installation.
It is a network only plugin and will only appear in the NetworkAdmin->Dashboadr->Plugins screen
Depends. If there are categories on the sub blogs before activating this plugin, They won't be available. The main site categories/Terms will remain available.
Yes. It works for custom taxonomy terms too. Just make sure the custom taxonomy is registered on the sit/blog you are trying to access.