Hello, Gerald!
First of all - you can't mix X450-G2 and X670-G2 in stack.
But if you will use other types of switches, if you don't care about hardware tables performance - you can make it. (If it would be just L2 cluster).
But you need to pay attention that if you mix, for example, X670 and X460 then numbers of HW tables will be reduced to X460 numbers:
You can check stacking possibilities here -
https://stackingtool.extremenetworks.com/StackingTool/
Thank you!