Q:

What can be done about a tag which is a complete subset of another? 

The physics-careers tag contains almost all questions also tagged soft-question.  Those which don't are deemed off-topic and closed. 
Most of the questions ask about the details of working in some specific field.  For eg.   

Current scope of Chaos theory and non-linear dynamics? 
How can some-one independently do research in particle physics? 

So do we really need a specific tag about physics-careers?  If so, what kind of unique questions would this tag describe. 

A:

In general there's nothing wrong with having a tag that is a subset of another.  After all, we do have broad subfield tags like electromagnetism and quantum-field-theory that many other tags will fit under.  It's still useful to have those more specific tags to make it easier for people to filter posts as they like. 
In this specific case, physics-careers questions may well be off topic on the site.  I don't know if we have an explicit discussion about it on meta but I don't believe there's any particular justification for them to be on topic.  That's a separate issue.