The first time I installed slack, I've installed everything.. you just don't want to miss important things. Slackware doesn't protect you against omitting dependencies, and this allows you to do the installation yourself later (from source for example) But I like the fact that you'e willing to learn a lot; just don't make it too hard for yourself. Slackware will learn you a lot already
Last time I've installed slackware somewhere else (as server), I knew what to remove. I don't think it's easy to create the best minimal install for the first time.
The entire install of slackware 9.0 costs 2GB of disk space. Note that it doesn't matter how much you install! It won't slow down your system; there are just more files. The real system slowdown is caused by starting too much programs (and network/background services) and running a big kernel, with much features you don't have/need.
installing extra packages can be done with the pkgtool (shows a menu), "installpkg <name>", and swaret (install packages/patches from ftp mirrors). If you forget something, it isn't hard to install it later.