A first consideration should be to ask if the cameras in question are modern 1000+ shutter speed kit or, if any of the MF (and 35mm) is limited to one five hundredths if a second?
I feel the slower the shutter is, the higher number of F-stops the ND filter should cover.
I.E., a Hasselblad 'V' series lens when using "normal" ISO speed films or faster, can really use such a filter if ISO 200 speed or greater is in the magazines you are using on a sunny day.
Stepping down to half that density, to five stops should cover the range for a basic set.
I do agree, smaller ND filters are needed, but for slow speed shutters, go big, (ten stops) or go home for a first step.
Colour films in general, IMO, do well enough with polarizers for most films, though F-stops choice may be limited.
Either choice for B&W or Colour, demand top quality glass or gel filters, but do no choose by price, B&W, of Heliopan multi coated filters may be pricey, but they perform at a top level and that's good enough.
Buy good used if you are able.
Forget variable NDs, they are poor choices for the dollar you spend and won't give you a full range of performance, based on other folk's reports.
Good Luck,
Eli