ps3ximconfig.exe is just a tweaked version of ximconfig.exe that shows ps3 buttons (sort of) rather than xbox buttons.
you don't need it.
yes, default.xim is what's loaded when xim.exe starts.
yes, use ximconfig.exe and manager to swap.
(btw, if you pretty much play cod4,5,6 or halo3 only, make that your default.xim file, obviously...)