Current policy is to use medium tags like poetry only for general questions about the medium (e.g. When did men dressed as women stop being the norm in English theatre? has the theatre tag) or for questions about works too short to merit their own tags (e.g. aurora-leigh gets its own tag, but questions about short poems are tagged with poetry and the author tag instead).
So, use the musicals tag if your question is about musicals in general, but use (for example) a the-rocky-horror-show tag instead if your question is about this particular musical. (I don't know whether there are any musicals too short to merit their own tags - if there are, then questions about them can also use the general musicals tag.)
Tagging works best when it can emerge naturally. While the community should step in in instances where tagging practices arent being useful, trying to plan tagging practices from the beginning is usually a bad idea. If you let people just tag questions and create tags when its intuitive to do so, then what happens is you'll get tags that people use and that are intuitive, even if they are sometimes logically inconsistent. If you try to plan tagging on meta, then you run the risk of creating a system that is logically consistent but not useful.
As usual, there's something to be learned from the RPG Stack Exchange:
Tagging is an emergent folksonomy. It does not need to be a strict logically correct hierarchy and overlap between two tags is not something we must do something about. Let people use the tags they find helpful and make sense to them. Overly curated tagging is IMO harmful to the point behind them and turns it into an advanced-users-only Dewey Decimal system instead of just letting people use terms that are meaningful to normal players.
In other words, this hypothetical tag applies to exactly zero questions right now. If we get musical questions and you think a tag would be useful, create the tag. We'll then be able to see how the tag works in practice and whether it is actually intuitive and useful.