I can confirm. We do have a customer running RH 5.x, even the uncommon 32-bit version. It is supported with disabled selinux and with appropriate version of glibc that comes with RHEL 5.4+
I would give it a shot with the Oneagent, If you have an older version you could even give that a shot. If those options fail you could always try agentless monitoring. Its worth a shot, it might work and it might not.
yes there is, Here is a great tutorial on how to set up an Agentless Monitored Device (AMD):