Hello,
Apple monitor usually work at frequency higher than 31KHz. You will not be able to display Amiga native 15KHz PAL or NTSC modes with it. It will work just as a VGA monitor.
Concerning the DB15 connector used in old macintosh monitors, it holds additional signals used by the mac computer to identify the type of monitor it is actually hooked to. You should not consider these signals if you use if with your Amiga. DB15 to HDB15 adaptors are available, but pinouts for such adaptors are widely disclosed on the net, and not hard to manufacture yourself...
Alma96