One must verify that the Operating System you wish to run is supported on your UCS hardware.
You can check this via the Hardware and Software Interoperability Matrix which contains a Operating System Interoperability Matrix.
Review the linked document for the latest information, however for the B200-M1 the supported operating systems are currently.
Is it safe to say that any distro based on RedHat will work? For example, will CentOS be supported?
Well you say "work" in the first sentence and "supported" in the second but they usually mean different things.
I suggest that other OS environments will work, as long as you can get the drivers for the special components such as the networking elements for the CNA. Cisco may support you on another OS to a point, but without it on the list that support will run out at a certain point.
I don't work for Cisco, I suggest you contact your local representatives if you have a real concern. They probably seek feedback on which OS vendors and versions should be added.