I second @ducktaperules
Mixing hardware is fine.
Mixing 4.x and 6.x is fine.
Mixing BLDC and FOC mode should be fine.
Mixing sensors, unsensored, encoders, and HFI is fine.
You should even be able to use different batteries/voltages for the different ESCs as long as all the ESCs share a negative connection.
I don’t think you can mix a duty cycle control with a current control scheme. (esk8 folks don’t use a duty cycle control mode)
You can use a VESC 4.12 and a VESC 6 and a FSESC 75100 together if you want with three different motors all with different Kv, different gearings, and different size wheels or propellers. It will work.
What you can’t do is mix firmware versions. All devices need to be running the same version of firmware, even if they’re different hardware files.
You also should not run a VESC 4.12 at 12S voltages in FOC mode, it will probably blow up. 10S max or BLDC mode.