My hooter with the 28 foot luff has a foot of around 14'. You might be looking for something closer to 24'luff and 12' or 13' foot.
As pointed out, a genoa off a monohull won't be a good match. You really want a sail that's designed to furl, and it will be flatter than a genoa, both to reduce the drag that will be generated at the cat's higher speeds, and facilitate smooth furling. It's more than just additional square feet of sail.
And yeah, it sounds like you're looking at monhull spin poles. For an 18 foot cat all you need the lower 12 some feet of stout windsurfing mast, or a 2' dia aluminum pole. It'll be attached somehow to the main crossbeam, supported and centered below the bridle wires (I used 3/16" Dyneema), and the tip of the pole will have wires run back to the bows or bridle tangs.
PM me, I think I have some pics of the SC20 set up.

Dave