After one week of testing using multiple datastores with VMFS 5 shared between an esxi 5.5 GA (build 1331820) cluster (with VMware vCenter Server 5.5.0 build-1476327) and one esxi 6.7.0u3 host didn't found any evidence of problems.
With this suggested configuration you lose HA and you will use an unsupported version of ESXi. Personally I would not use this as my production environment.
Thank you for the advices.
For the esxi 5.5 version you are right, it's unsupported but unfortunatelly a full migration it's not feasible yet for us.
As I will keep completely separated virtual machines, except residing in the same datastore, inside 5.5 cluster and 6.7 host (that will be probably in future inserted in a different 6.7 cluster, but for now we well only keep VMs with no HA needings on 6.7 host) I want to ask you a clarification. Do you think this will break current HA on 5.5 cluster or do you meant that you cannot have HA between 5.5 and 6.7 hosts?
You will have no HA on that single 6.7 host. HA on the 5.5 cluster will not change.