Jeff,
In a typical ply hull the choice of ply or foam usually depends where the bulkheads are in the boat.
Usually the bridle (on a Stringray it would be where the forward beam is), Main beam, Rear beam and around the centrecase are ply.
Only use gaboon or Okume ply to BS1088 standards and coat in two coats of epoxy and the bulkheads will not rot. When treated properly with expoxy ply bulkheads will last forever inside a hull. I had a set of hulls filled with water for nearly 7 years and when I took the decks off the inside was still like brand new.

As far as the other bulkheads, they are usually polystryrene foam and 50mm thick. This is beacause the polystryrene foam weighs 17kg per cubic metre and is the lightest by far of the foams. The reason 50mm is used is to provide some floatation. THe polystryrene foam will absorb water over time but if the inside of the hulls are kept ventilated that should not be a problem.

As far as the foam your hulls are built from:- just to give you an idea of weights probably the most common is 80kg per cubic metre although it comes in a range of densities) it can be used in load bearing bulkheads same positions as ply already mentioned but would need a proper glass layup (preferabely under vacuum) to make them strong enough to do the job. So you don't want 50mmm thick foam in that density.

Having said all that I have not gone through a set of Stingray plans but I don't think there is much at all in the way of bulkheads. Partially because with the 3 beam platform some of the hull loading is taken by the forward beam.

I expect your plan to replicate the faom in both hulls will do the job of assisting with some of the compression experinced by the hull sides in rougher conditions and the foam will be lighter than the ply. Sealing the foam with epoxy is a bit of a two edge sword because if water does make it's way in you will find it harder to dry out. It's easy to seal ply properly with epoxy but not so sure about the polystyrene foam you are considering because of the way it is made.

Personally I'd probably check the keel and maybe add another layer of tape if that in an issue and then use foam and make sure the rear hatches are left off when not sailing to keep the hulls ventilated.
Just the way I see it.

Regards,
Phill