You can vMotion between ESXi 5.x and 4.x, and mixed clusters are fully supported. Caveats being that virtual hardware 8 and VMFS-5 are not supported on ESXi 4.x. Source: Mixing ESX/ESXi Versions in an HA/DRS Cluster
I don't see why you wouldn't be able to add an additional LUN, as long as you use VMFS-3 if you want it to work across versions.