From what I've seen there's little ryhme or reason to freewheel sprocket hub sizes. I've taken a few apart and found that none of the ones I did were compatible other than two Shimano ones of the same era. So by the time you find a freewheel that IS compatible you may as well just spin it on instead of messing around swapping sprockets. The only possible case where this would not work may be with a 7 speed Shimano where the sprockets may be the same size centers.

Often there's "almost" enough room for the 7 speed. If you were to play with the spacers on the axle and shift them around a bit or introduce new spacers you can offset the hub a little more to allow using a 7 speed and then adjust the non drive side spacers to get the overall axle nut spacing back to where it is supposed to be. From there adjust the wheel dish to recenter the rim. As I understand it this is exactly what they did when the makers switched from 6 speed freewheels to 7 speed freewheels so you're certainly not doing anything unusual. It DOES make it so that the drive side axle has more unsupported length and on bikes used for serious trail riding this can lead to bent axles. But if it's more of a grocery errand bike it should be just as fine as all the other 7 speed freewheel bikes in the world....