there are lots more than even those 2 standards. yep, nema and the metric is called IEC sizes. I have experience with thomson micron & stober gearheads: both will mount to ANY motor - within reason. there are limits to min and max shaft diam and then of course flange size must be adaptable. these and others like alpha typically just ask for the exact motor part no customer plans to use with a given bo and then provides that adapter plate for typically no additional cost. if one gets hold of a used box then of course they will have to pay for the adapter plate and coupling piece(s).
any distributor for your brand gearbox should be able to get you a part no and price for your exact motor if you ask them.