I administer a medium sized Linux cluster running RedHat 7.3 . This product will soon be end-of-lifed. I would love to upgrade the entire cluster to Fedora, but the starCD package that we run is incompatible with the version of glibc shipped in RedHat 9 and newer. Would it be possible for me to create my own "custom" distribution with all of the Fedora packages except for an older version of glibc (say from RedHat8) and a more generic kernel? Has anyone tried to do this? If I re-burn Fedora CDs with my own custom glibc and kernel RPMS - will that work? Thanks for your help.

Would it be possible for me to create my own "custom" distribution with all of the Fedora packages except for an older version of glibc (say from RedHat8) and a more generic kernel?
I think there's two options to explore before you try what you suggested:
I. Check for compat-glibc rpms, or
II. Try installing Fedora Core, then relocate and install RH7.x' glibc rpms to, say, /usr/lib_rh7 and load that with LD_PRELOAD or LD_LIBRARY_PATH before starting the app.

Needless to say I'm totally unaware what you mean by "starCD package", posting more verbose nfo would have been beneficial.

I'm sorry if I was a little vague. StarCD is a CFD (Computational Fluid Dynamics) program that is put out by Adapco. The program runs caluclations in parallel on the cluster nodes and has a rather complex GUI that runs on the server. Unfortunately, Adapco does not support any RedHat distributions beyond RedHat 8 because they contain newer versions of GLIBC. If I could somehow meld RedHat 8.0's GLIBC and kernel with the rest of Fedora - I would be happy.

OK, that's clear. BTW, I forgot about the NPTL stuff, and I think you won't find a compat-glibc for RH =< 8.
Still you've got option II to try out.