No, Linux clusters don't specifically require RDM.
Most Linux clusters use cluster filesystems such as Lustre or RedHat GFS. Generally, your "front end" (host) nodes will communicate with a storage node via a network transport (ie. iSCSI, GNBD) so an RDM mapping isn't completely necessary.
So can I start creating a cluster-in-a-box with 2 VM on the same ESX host using a "shared"" vmdk on a SAN DS and later , when a second ESX host is been available vmotion one VM to the second host ?