I seem to be on a one-man campaign lately, on several forums, to teach strip builders that a weak inside layer makes for a weak boat. You're building a sandwich which acts somewhat like an I- beam. If one side of the sandwich has half the strength of the other, or if the top plate of an I-beam is only half as strong as the bottom plate, the resulting construction is nowhere near as strong or as rigid as it would be with equal strength on either side of the core. This gets even worse when you apply it to the bottom of a canoe or kayak where the vast, vast majority of the destructive forces it may encounter come from the outside, pushing inward, compressing the outer layers of fiberglass and trying to tear apart the inner layer, usually breaking the core in the process.
In normal situations, the outer glass layers on a stripper don't do much more than keep the rocks from getting into the core. The sturdier and thicker you make them, the harder you can hit a rock before having to do a serious repair. The inside layers, on the other hand, don't get abraded like the outside may, but their function is far more important - they keep the boat from breaking! - whether from impact or just the force of the water pushing up on the bottom, trying to oil-can the hull.
A strip core is extremely unidirectional and has very little cross-grain strength. Upon impact or when the boat hits waves the outside glass layers are not in the position where they can do much reinforcing. You can split the core wide open, blow apart the inner layers and the outside layers will have gotten so little strain that they may barely even show a fracture. Whether or not the core splits and whether or not the inner layers rupture is strictly a matter of how strong the inner layers are. If in doubt, always balance the layers, inside and outside and be very aware that reducing the layup on the inside of a canoe bottom makes for a much weaker boat. Why build a boat that you know is weak? - to save a few pounds? - Not worth it in my book.
Bulkheads are a double-edged sword. They are exceedingly practical, especially on a sailing canoe if they're helping to form watertight flotation chambers or reinforce mast partners, centerboard trunks, etc. They do tend, however, to make stiff spots called "stress risers" in the bottom of the boat which can tend to focus abrasion and/or impact damage in the area around the bulkhead. On a sailing canoe the stress riser part of the equation probably isn't worth worrying about unless you plan to do some "whitewater sailing"....
A single layer of six-ounce is probably fine on the outside of the deck and will yield maximum clarity on this very visible part of the boat. I think I'd probably use the same inside, but back it up with some glassed-in ribs or deck beam structure in case anybody ever sits or kneels on the deck.