IMO there's no reason to use RDMs anymore except for specific requirements (e.g. MSCS). Using a virtual disk rather than RDM will also reduce complexity, i.e. you will have less presentations from the storage system. Btw. in case you are going to use RDMs anyway using Navisphere Express, make sure all hosts recognize the RDM with the same LUN-ID.
Btw. why do you want to use a dedicated datastore for the data disk? I'd simply place the data disk and the system disk in the same datastore folder. This again reduces complexity and you don't have to maintain free disk space on several datastores for snapshots. A file server's system disk doesn't usually have high IO requirements after the system has booted.
André