Can I replace a MBP 2012 SSD with the Flash storage from a MBP 2013?
Out of the box, definable you not.  The 2013 MacBook Pro laptops use a proprietary PCIe interface whereas the 2012 used a SATA III interface.  

There may be a PCIe to SATA adapter, but I haven seen any that are compatible with the Mac’s proprietary interface.  Even so, SATA SSDs are very cost effective right now, so it’s not practical to add that extra layer of complexity by putting an adapter into the mix.