Is 4S+3S in serie possible?

[MetalMan]

For starters a 2700 kv motor on 6s is a very power hungry setup and generally not recommended at all (except maybe for speed runs). But if you change only the motor and not the gearing, it's gonna have less power with a lower kv. Bigger motors are capable of handling more power and will typically run cooler than a smaller motor at the same power level.

If you changed the gearing of the 1900kv to match the top speed of the 2700kv setup, the 1900 will run cooler (even if the motors are the same size).