firmware upgrade and compatability questions

we have mix of fortigates, 500e at main office, 81e and some 90ds at remote sites, fortianalyzer VM.

managed individually, no fortimanager.

currently all fortigates are at 6.0.10 or 12.  we have ipsec tunnels between the 500e and all remote fortinets.

question:

are there any communication or compatibility issues between fortigates running 6.0.10 and those running 6.4?

the 90ds cannot go past 6.0.12, but we are looking at replacing them later this year.

and if not, should we upgrade the 500e to 6.4 first, then do the remote sites?