Why are you tracing over a manufacturer detail?
Sorry if that seems blunt...
But there is a workaround to this if you must have lines smaller than 0.8mm for details.
Open up a new family.
For 2D's recommend detail components template.
Import your CAD detail directly into the new family template.
Make sure it is at 1;1 scale
At this point, I would do a first save but remember to resave after you use the convert line type to a detail line.
Now you have a detail component that is your manufactures detail with lines below the 0.8mm minimum that Revit normally applies to lines.
Things to note
First, of all, you shouldn't be doing details in less than 0.8mm with detail lines and fill regions.
In my opinion, I feel these should be used in a bare minimum, preferably not at all.
Second, the detail component method I explained above will not let you edit it.
This is because Revit will apply the 0.8mm minimum, any changes to that detail will have to be done in AutoCAD. and the whole process will then need to be redone to create a new detailed component with the new sizes.
Lastly, I would suggest more generic items as we all know clients and contractor like the cheapest/ best deal products they can get there hands on while still having the same specification (your manufacturing detail won't be relevant if this happens)
If you need an example of this just request it. with a reply.
Hope this helps you out