Yes, pe-i386 and pe-x86-64 are the respective 32-bit and 64-bit object formats.

Your commands seem reasonable.  With gendef, I just let it create a .def file with the same name (i.e. skip the '-' and redirection); in my mind, that reinforces the association between the dll and def files.  With dlltool, I did not have to use the -m flag (as x86-64 is the default for me), but I don't see anything wrong with being explicit.