how about forcing rpm to accept an older version?? "rpm -i --oldpackage krb5-libs-1.2.7-10.i386.rpm
" This should install the rpm without an error saying that there is already a newer version installed.
also, if this does not work and indeed all you need is this single .so file, you can also extract the file from the rpm and drop it into your system (dirty, dirty, dirty hack, but works wonders). The command would be rpm2targz
(make a tar.gz archive with all files contained in the rpm. You can easily extract this single file from there). I dont know if that is installed by default though.
Also, are you certain that you are installing a recent version (depending on old libraries seems odd)?
I am sure there are reasons you do not want to use cups. However, if lprng does not work for you, you might want to give cups another chance.