If the sail is shorter on the forestay than your existing genoa, just fit it to the tack and raise it normally. The head of the sail will simply be lower at the top and that isn't a problem.
It'll just be a smaller sail. Something you'd want in heavier air anyway lacking roller furling. I had 120%, 100%, 70% and 50% jibs on an O'Day 22. The 70% and 50% jib were smaller jibs that I bought for heavier winds and just raised them to their limits.
Use the jib halyard to supply tension. Raising the foot of the sail will affect how you trim it and it will be easier to trim and maintain airflow with the foot in the correct position. Worse that happens with the shorter jib is you'll be acting like a fractional rig rather than a masthead (assuming your boat is a masthead setup based on your description). Won't affect sailing at all except to depower the boat.
SV South Paw
2000 Hunter 340
South River, MD