Are basic system administrator utilities such as useradd or adduser standardized? If so, where can I find the specs? (POSIX doesn't seem to encompass those, but I might need to take a better look).
No, these utilities are not standardized. A quick look through the
useradd(8) manual on RHEL6 versus OpenBSD reveals that while there are similarities, various flags differ in purpose. For a broader view, http://bhami.com/rosetta.html lists under "managing users" a variety of different commands, depending on the particular flavour of unix.
You can use
man useradd and
man adduser to read manuals of each of these utilities to get an idea about how they work. AFAIK, there is no "standard reference sheet" for these utilities.
You might also take a look at shadow page, where you can find sources.