The nikon fm doesn't have ttl flash so any old fashion flash would do.
the general guidelines do apply:
* bounce flash better than fixed
* flash with pre-set apertures would be preferred, eg. iso 100 and f2.8, f5.6 and f8
* flash with thyristor would be great as the flash will meter the environment and flash the appropriate strength
* flash with adjustable flash strength would be preferred
I have had success with "Leica CF" flash built for the minilux, also used sunpak flash....