You would set your line thickness as part of the line style. Keep in mind that the thickness is defined in plotted units regardless of scale. If you need several thicknesses (I assume for each rating value) then you would just create a new line style for each thickness.
I wasn't suggesting you create the detail component in a separate file. Here are the steps I am suggesting
- Duplicate your floor plan and name it Level 1 Fire Safety Plan
- In the duplicate plan place your repeating detail 'lines'
- Create a sheet view and place the Fire Safety Plan first
- On the same sheet place your first floor plan with just walls, doors etc. and align it with the other plan view
- Set this view to be wireframe and the filled regions should be overlayed on the walls and allow the wall to show through.
After all this is done, I agree, being able to just add a line with a transparency, or better yet add a line to the wall definition, would probably be easier.