Check this most updated Guide [URL]  it seems that in my case 9.3.5 is fine:  NoteOnly update firmware with Node Firmware Package 10.0.1 if your cluster containsGeneration 6 hardware or Gen 5 with 40G Ethernet cards. If your cluster does notcontain Generation 6 hardware or Gen 5 with 40G Ethernet cards, update yourfirmware with Node Firmware Package 9.3.5.  Thanks for your help
Hi,  I am looking for which prerequisite a Gen6 node request to be added to a Gen5 cluster.  Do you know whether a specific node firmware is requested?  BR
Isilon Gen6 Prerequisite