Here is my caveat: You said you already verified support for your server for the CUCM version you want to load. I'm taking your word on that. You also need to verify that you have the proper CPU, memory, etc. based on the hardware support guide.
With that said, you should technically be able to set up a cluster with any server that supports your version of CUCM. Now, here are the caveats with that:
1) You cannot promote a Subscriber or change the Publisher to a Subscriber - not possible. You have to rebuild the cluster to do this. So, the Publisher you have now is the Publisher for your cluster.
2) If you install a server that is technically more capable than your Publisher (or any Subscriber for that matter) - ex: 7825 is current Pub, you install the equivalent of a 7835 or 7845 - you still need to plan capacity based on the least capable server. Vice versa, if you installed the 7816 (not saying it's supported and I would never use one in production personally) then you would need to consider the cluster limitations to primarily be based on the specifications of the 7816.
3) If you are using NTP, make sure you verify via CLI that the Publisher is sync'd before you try to add the Subscriber via the installation process. If it is not, the Subscriber installation will likely fail as it's hardware clock is set to that of the Publisher.
That's about it. Make sure you flush out the HP server and verify the specs on it before you proceed...i.e., save some headache for yourself. As far as MCS branded vs. HP direct servers, Cisco servers are a simply a specific OEM build of an HP server. Nothing special there. Same with IBM.
Hailey
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