Just a quick installpkg question
 

Does installpkg work for any .tar.gz source file, or does it have to be specifically prepared to work with installpkg?

Like, will

install whatever tarball I happen to download somewhere?

at my work computer so I can't check for myself atm.

No. Installpkg works on slackware packages which are compiled binaries, not source. Run a slackbuild script and it will build the package. Then you use installpkg.

It has to be specifically prepared to work with installpkg.

The program that does what you're describing (build source packages into Slackware packages) is src2pkg.

is src2pkg in slackware 13.1?

so
will install the software?
no need to do the manual make install thing?

That will work, but it's always a good idea to have a look on slackbuilds.org to see if someone has made a slackbuild script for the software you want to install already. It's always better to do that rather than use src2pkg, as good as it is. JMO.