Hi
Since there is no such thing as a 750 power supply for the 3750/3560 I will assume that you mean the 350.
Yes you can have different powersupplys in the same switch, they do not need to be the same.
I have a stack that runs with 1 350 and 1 715 powersupply in each of the switches.
There are the following powersupplies
C3KX-PWR-1100WAC=
C3KX-PWR-715WAC=
C3KX-PWR-350WAC=
C3KX-PWR-440WDC=
One thing to make a note of is that the 715w ps has a different type of powercord connector than the "normal" powercord.
Good luck
HTH