Transfer scale max & min parameter from variable to a modbus slave?

I wouldn't say best practice as I have never encountered this being done in my experience.
Cicode would be you best friend here, nut I would have to ask why you are doing this. I would assume all scaling is in the Modbus device.

Yes, scaling will be done at a low level, but with values defined in (if practical) Scada due to some limitations with access to the modbus slave.
This is in case there is a need to replace a sensor or any adjustments. 
I know I can only read raw values from the MB device and do the math in Scada, but not for this project as there are several devices that need scaled value from the modbus slave.

A practice I have seen used in situations like this is to create a new variable that will specifically store the scaled value of another variable. This approach will take up another I/O point against your total I/O point count, but if you have plenty of extra points, and depending upon how many scaled variables you want to do this to, it is an option that would circumvent additional backend processing to scale the variable in question.