From what I have gathered, the 900SS fronts were 17mm axle from 1991 to 1993. 1994 to 1998 SS front went to a 20mm axle. Rears on 1991 to 1998 SS bikes are all 17mm axle.
1999 and newer SSie bikes use the 25mm Superbike hollow front axle. Not sure about the rear, it may still be 17mm.
As far as Monster vs SS compatability, no idea, but I believe a fork swap is required to use the modern 25mm front on a 1991-1998 SS bike.
What the hell, you own an FE, spring for some Marchesini's
Hope this helps, Ed.