Overall yes a 115 mm brake will be too big on your 90mm ski. However FKS do suck for changing brakes. If you cant find some spares and someone to switch them, you have to get rossi to do it, for $175, which makes that good deal less then good.
Maybe see if you can find either an FKS with smaller brakes or just find any other binding which you can switch the brakes on.
For the other guy above. Probably use the 100mm brake, its better for them to be a little tight then too big, genrally. We were using 115mm brakes on teh Rocker 2 which was 122mm underfoot and it was working out well. The 130mm was slightly better in that situation, but I would use the 100mm in your case.