EQ Looks to the designated UISKIN (vert, quartz, myui, etc) directory
first and then the Default folder. Anything NOT in that designated directory it will THEN PICK from the Default folder.
For
every file (with the single exception of the EQGUI.xml, I believe..) it must pluck from the Default folder it generates an 'error message' in the UIERRORS.TXT file. These are not actually errors. The fact that is lists them, to me, makes no sense whatsoever.
So, yes you can mix and match to hearts content as long as your mixing and matching things dont overwrite each other. EQ will find the rest.