Hi TBird,
There is no waterline length problem. If you use a spin pole that is 40 to 45% of the mast height, this will lay the spinnaker back so that the resultant force from the spinnaker will lift the bows and reduce the pitchpole tendency. Again, if you want a spinnaker launching system that works well right out of the box, GO TO THE FACTORY. The Aquarius-Sail factory supports the SC boat owners with normal upgrades and modifications to the SC product line of boats, all sizes. No other factory does this.
Yes, I have seen a spinnaker on a SC15.
You never sail a beach cat on a "downwind run". That is the slow way to go downwind; this is monohull sailing. To get to a point downwind on a sloop rigged beach cat, you sail downwind to a relative wind angle that is approximately abeam. Trim the mainsail and jibsail out for reaching downwind but in just enough to get the tell-tales to fly on both sides of both sails at the same time while sailing as deep as possible. Once the sails are set and tell-tales flying properly, cleat the sails, don't change them, and keep the tell-tales flying by steering the boat only. It is something like sailing to windward, steer to the tell-tales, except you are sailing to a maximum downwind angle instead of a minimum upwind angle.
Bill