Veronique,
From a technical PoV, you can run CA Spectrum under CentOS - in the same way that RHEL runs. This includes having OneClick/MLS/LS on any combination of OSs and versions. However, I would highly recommend that for management purposes that you stick with one OS (better yet, under a supported OS). That way this eliminates the chance that issues that you may see are not OS related. Also, having the same OS makes patching a lot easier!
Ed