Yes it's compatible. Both Cmd 6.2 and Cmd 6.5 should be compatible witch ExtJS 5
You can always install multiple versions of Sencha Cmd on the computer. But you can have only one in the path (in the terminal) but you can always execute the second directly from the executable or create another path with different name.
Also Sencha Cmd 6.x provides nice command sencha switch where you can switch the version on the fly.
Code:
> sencha switch --list
Sencha Cmd 6.5.0.180
Looking for versions at: C:\Users\Petr\bin\Sencha\Cmd
Current version
6.5.0.180
Newest version installed
6.5.0.180
Locally available versions
5.0.2.270
5.0.3.324
5.1.3.61
6.0.2.14
6.1.1.76
6.1.2.15
6.1.3.42
6.2.0.68
6.2.0.73
6.2.0.75
6.2.0.78
6.2.0.103
6.2.1.29
6.2.2.23
6.5.0.168
6.5.0.176
6.5.0.180
Code:
> sencha switch 6.2.0.103
Sencha Cmd 6.2.0.103
Successfully switched to version 6.2.0.103
Regards
Petr