First of all there are MANY ways to isntall software.
IMHO, the best and cleanest way would be to use Slackware Package Manager. Among a few cmds available under this umbrella are, pkgtool, installpkg, removepkg, rpm2tgz, upgradepkg etc.
Often, there are software which you wish to install but they are not compliance with Slackware Package format. And they have their own installation method, for example Acrobat Reader from Adobe. It is perfectly alright. Because basically it is just copying the files to the correct location(s) and also setting up the right settings (maybe).
The minus point of not using Slackware Package Manager is when you want to remove the software. You may need to hunt for it.
As for keeping Slackware up-to-date, you can subscribe to the mailing list for updates at
www.slackware.com. Or you can check out the slackware-current. Download the new packages and do upgradepkg xxx-i486.tgz. It actually removes the old one and install the new one. You have to check for the patch command if you just want to patch from the Slackware9 folder.
RPM stands for RedHat Package Manager.