If possible, I think we should treat the :foo in /tag/:foo as always being a slug, and require the form /tag/:foo/:id for IDs, where :foo can be any character (e.g. use - when you don’t know or don’t care).
This would help for backward compatibility and also in cases when the slug in known, but not the ID, which would often be the case for tags, where the name is the commonly used reference.
IDs are really there now primarily to support localization.