Jurassic Carp, Fat Bomber, The Tiger and Red Alert are all ply boats with spinnakers. There may be others I've forgotten.

There are no guarantees. Bows break off old ply boats sometimes (like Nick's) without needing a spinnaker to help them. If the boat seems solid and the ply does not flex (or worse, "squish") when you give it a push then it should be ok.

If you break it with a spinnaker then it probably would have broken anyway.