You can comfortably leave the deck beams to minimum.
In cedar canoes the gunwales are in two pieces, one inwale and one outwale, they catch in the middle the ribs and the cedar skin. Effectively the cedar skin and the outwale can represent a lamination by itself, that already tends to keep the shape a little but the fact that the ribs are caught in between that and the inwale builds a really rigid structure (in addition the holes left in between the ribs on the gunwales work as scuppers when the canoe its upside-down on the dry), the decks (breast hooks) do the rest.
You say its not your plan, but if you build that way eventually adding some spacers if you see that the ribs arent as consistent as in a canoe construction (or if you dont have ribs at all) they will keep their shape nicely, if you want to overdo it build each inwale and outwale from a 2/3 laminations. Otherwise, if you absolutely need to not have an inwale/outwale, build up the whole gunwale in several laminations (dont forget to make an hole here and there for the water to come out) but that would be heavier, more work and actually less rigid than the other system, you can even beef it up using a fiberglass ribbon in between the strips when laminating.

The measures you have in mind would be ok for a normal canoe (15ft) and made by a 5/8 inwale a 1/4 ribs spacing and a 5/8 outwale planking included (Canoecraft?). For a Wee Lassie size built like an SOF id use smaller stuff. Eventually with those measures try to save some weight building a corner like an upside-down L where the inwale is the vertical piece and the outwale the orizontal but they are laminated together to form a single piece (switch in-outwale to your liking)

If i can give a suggestion, unless you plan of putting a deck, full or half, let the tops off as much as possible, it gives more freedom, keep instead a sort of yoke that you can use for portaging and attaching stuff (fishing rod, short mast maybe) that will be attached with screws or clamped to the inwale, through the scuppers, in that way you can move it where you need it or take it off completely.