Hi
On the vSphere/ESXi side yes you should use the HPE custom images that can be downloaded from the HPE vibsdepot here: HPE Software Delivery Repository vibsdepot (aka HPE Online Depot)
On the firmware side, follow the recipe document: http://vibsdepot.hpe.com/hpq/recipes/HPE-VMware-Recipe.pdf
On the physical switch side stack is not mandatory, you can use route based on physical NIC load (Load based teaming) or route based on originating port ID so that ESXi utilise multiple uplinks/vmnic.
In IP storage such as iSCSI setup, you would use multiple VMKernels and iSCSI multipath/portbinding which does not require the physical switch to be stacked anyway.
Based on this link, x4012 supports Jumbo Frame up to 9,000 bytes: Dell Networking X-Series Smart Managed 1GbE and 10GbE Switches | Dell United States