initrd is generally used when you don't have essential system components compiled in as kernel modules. Root filesystem (as a module) comes to mind immediately. I've never made initrd with slackware and i've never had a problem before. Essentially I compile root filesystem in the kernel...also i've always compiled cpufreq and acpi as modules. Try making a kernel for your system taking the default 2.6.13 kernel config file as a starting point. Compile the filesystem (reiser or ext3 whatever you are using). Once you successfully compiled you own kernel...you have the .config file which can be used over and over again.
Now the question is can i load acpi modules and cpufreq scalling modules with an initrd image if yes how?
As I said, i've never used initrd so I wouldn't know if one can load acpi and cpufreq via initrd. I always load them via /etc/rc.d/rc.modules script. I add those entries at the very end of the file and it always works for me.