1. Someone has made Slackware packages for GTKam and libgphoto2. Check
LinuxPackages. The GTKam package was made for Slack 9.0 but should work on 9.1.
2. This might cause problems. If one of these programs needs a certain library then it will look for it on your Slack machine. That's fine if you have that library, but bad if you don't. Basically the program won't run in that case.
3. Don't worry about where to put the source. Just untar it wherever, run './configure --prefix=/usr; make' and then instead of 'make install', get
checkinstall. It does what 'make install' does but first makes a Slackware package so uninstallation is easy.