Having one file for developing your standard details is a reasonable approach. You can publish these as modules to easily insert them into the project file (or copy/paste if you have both files open - multiple users can open the standard detail file as Read Only).
I don't recommend keeping all your developed project details in this one single file. While there are ways to make this work (Teamwork for multiple access, ID codes by project, etc.) but it is asking for trouble. There are so many possible problems with this that I can't go into it right now.
You should have your attributes sufficiently standardized that details can be imported to project files without problems. Pens, fills and linetypes are all that really matter. Details can generally exist on a single layer and variant pens can be handled with pen sets if necessary. So if you are standardized on your line and fill types you will be fine.
