I don't recall anyone on the forum fitting 15" wheels to a 90 series cruiser , in theory it's possible as 80 series wheels are interchangeable (and use the same calipers) and some 80 owners have gone from 16 to 15 by way of using a -35mm offset steel rim IIRC .
I can't say for sure if it will work though , i had 16x8 steels with a -38 offset and when i looked at them while debating myself about trying to fit 15's it looked like it would be tight , ultimately because i couldn't borrow any 15's from anywhere to test fit i stuck with 16 .