New esxi 6.0 hosts attaching to older infrastructure

Hi

So we have bought some nice new R730, esxi 6.0 U1b hosts to replace some of our aging servers.

We currently have esxi 5.0 / 5.1 hosts - MEM 1.2 drivers attached to several groups with a couple of different firmwares.

 Am I correct in thinking, v6 FW isn't compatible with esxi 6.0 / MEM 1.3 and that group needs updating?  Go straight to 7.1.5 so they are all the same? I don't really want to have to do them all to the latest if not needed.

Once that is done, can the esxi 6.0, MEM 1.3 hosts quite happily share the same datastores as the old hosts with the older MEM 1.2 as we migrate VM's at our leisure? Any problems doing that?

Any other advice or gotcha's that I have probably missed?

Thanks