1. Is it possible to replace the kernel of one distro with another distro? (ie. Slackware distro with RedHat Kernel)
No you cannot, redhat kernel is made for the specific distro, I recommend you download a real kernel and compile it.

2. How about using one Slackware variant's kernel on another Slackware variant? (ie. ZipSlack kernel on my DosLinux)
I dont think so, but Im not sure.

3. Can I use an SCSI kernel even if I don't have any SCSI controller?
You can emulate SCSI by compiling the SCSI emulation option in the kernel and by passing the boot option in lilo: append = "ide-hdx"