You can install ArcGIS Server (10.1, at least) on CentOS 6 and it will run just fine. Whether you want this for pre-production is another story though since ideally pre-production and production environments should be identical. But since the purpose of CentOS is to be a near-identical replica of Redhat anyway, maybe that's good enough.
Edit: whoops, just realized the original post is a year old
You can install it, but it might not work. I had a customer who had wanted CentOS 6, but had to switch over to RHEL 6 when a security patch released just about one year ago caused ArcGIS Server 10.1sp1 to suddenly stop working. Upgrading to 10.2/10.2.2 didn't help. Replacing the OSes set the project back a week. Far better to use a supported OS than work without a net.
CentOS 5.11 continues to work for me, but I wouldn't recommend it for integration, testing, or production.