I have just finished (well, it is never going to be finished) a version of Slackware for CF cards, so I can give you some advice on this one.
If you are planning on installing Slackware on the CF like it was a harddrive, and then mounting it as root (hda1) forget it. Even though it could be done this way, it would be very slow. Also, the space on the CF card could be used much more efficiently.
For my system I boot into RAM, using the CF card to hold only the image of the Linux system (and kernel). This has two advantages, first, using a RAMDISK as your root mount point is going to be very fast, and two, you can compress your system image and get 3x the amount of data on the CF card.
For instance, I am using a 32 MB CF card which boots up my 28 MB Slackware system. But, only 12 MB are actually used on the CF card.