There is a way to boot a floppy that will turn over the boot process to the cdrom. You need to have a Windows machine running in order to do this. You can download the sbootmgr.dsk disk image from a Slackware mirror site to your Windows system. Then use the rawrite.exe tool on your Windows system to copy this image to a floppy disk.
I don't know if this floppy image will boot or if you need a Slackware boot disk. If you need a Slackware boot disk you do the same thing with the boot disk image. You would want to use the old_cd.i boot disk image. You would do the same thing. Download the old_cd.i image to a Windows machine. Then use the rawrite.exe utility to write this image to floppy.
More information can be found on these two pages.
The Slackware mirror sites have all got the same directory structure. I will show you where the files are in this structure. You can use a different mirror if you want.
The rawrite.exe utility is found at
The sbootmgr.dsk file is found at
The old_cd.i file is found at
Hopefully when you boot these images from your floppy disk then you will be able to tell the software to boot from the cdrom.