If you're going to use rpm to install packages in slack, you'll need to tell it to ignore dependencies with the --force and --nodeps options. RPM checks dependencies against its own database of installed packages, but it only knows about those that you've installed with rpm. All the *.tgz packages that make up most of your system are unknown to rpm, so dependency checks will almost always fail.
To be honest, I'd recomment using rpm2tgz or alien to convert the rpm to tgz, then installing it with pkgtool (installpkg whatever.tgz). That way, all your packages are listed in the same database, so they're easier to keep track of. Also, since pkgtool doesn't have any dependency checking, it will never refuse to install something because it thinks you're missing a dependency.