Just to let you know, there is an excellent search feature on the site, here's the link (and it's located in the upper right portion of the screen):
http://www.linuxquestions.org/questions/search.php
Now, your questions:
To make a program run at startup, place this program with full path in your /etc/rc.d/rc.local file, no special format, just /full/path/to/program -with-any-flags
System monitoring applet is most likely grkrellm,
www.gkrellm.net
And no you don't necessarily need KDE or Gnome to run blackbox, however if you are going to be running any KDE or Gnome apps in blackbox, then you just might.
Oh, how do you make slackbox run gui? Configure X first, use xf86config then after you've booted up, you can startx into a gui. If you want to start directly into a gui after you've determined X works, then edit the /etc/inittab file, change this line:
id:3:initdefault:
From the 3 to a 4 (Slackware specific).
Cool