How to allow a specific "unknown source" I would like to be able to install applications from a specific "unknown source" (it's our company continuous integration server).  Is there a way to do that?  (Using a Samsung Galaxy 7 if it matters).
No, you can't do that.  As acejavelin says, "Unknown Sources is all or nothing, you allow everything outside of Google Play, or nothing."

Depending on your phone, you may be prompted to allow for just this install (for example, Samsung Galaxy 7), or you may have to enable, install, disable (for example, Sony Xperia Z1).  The former is obviously safer.