top mounted ballscrew, would it work properly ?

[m_c]

The more DIY methods might not require a bigger motor, however they're not recommended unless you want to be constantly having to adjust things to try and get consistent and accurate performance.
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Different size motors on each axis are not a problem. Whatever controller you end up using, is only interested in the number of steps it takes to move the axis a set distance. The controller is only responsible for sending the correct number of steps to the motor drivers, and has no interest in what the motor driver is actually powering.

[Boyan Silyavski]

The more DIY methods might not require a bigger motor, however they're not recommended unless you want to be constantly having to adjust things to try and get consistent and accurate performance.
.
Different size motors on each axis are not a problem. Whatever controller you end up using, is only interested in the number of steps it takes to move the axis a set distance. The controller is only responsible for sending the correct number of steps to the motor drivers, and has no interest in what the motor driver is actually powering.

In fact if a top notch machine is the final aim, there could be a problem with PSU choice. I was going to buy some stuff these days and almost for a moment missed that point.
Was looking to buy 2x 3Nm Nema 23 closed loop kits for x and z , plus 1x 8Nm nema 34 for the both long screws connected by belt. then i woke up and saw that the different sized drives need different voltage. This could be cured buying the intended motors but only with the bigger drives, which could drive the small motors. So feed all with same voltage.
For normal motors just consulted with Dean yesterday, and he confirmed me that to keep all same /speed, torque, acceleration/ and exchange 2 motors with 1, you need higher voltage for the nema 34motor. Basically in this case- 2 different power supplies. Which is not a big deal, just to have in mind