Wayne,
While it's a joy to build a scale model of a real boat, the realities of physics will quickly get in the way and the modeler will be challenged to make compromises to accommodate them.
I apologize if this is all known to you but if you don't make adjustments, a model yacht will be at much tippier than the full size yacht.
Stability is an interaction between righting moment (which is a combination of hull shape/weight, ballast weight and location) and centre of effort and size of the sails. Hull size and weight is a factor of its volume/displacement and centre of effort is a factor of the size and shape of the sails.
You indicated that you were building in 1/15th scale. Your sail area will be 1/225th the real boat (H X W) but your displacement will be 1/3375th the weight of the real boat (H X W X D) so the scale modeler must make adjustments.
For the large square rigged models, an extra drop keel is often added that hangs way below the true keel. It is removed for display. The higher square sails will not be set to reduce the sail area.
For scale yachts, slightly deeper keels (as you are planning) are usually used. The "J" class allows 2 extra inches. For sail area, slightly shorter masts and booms will reduce sail area without looking "wrong."
Traplet publishes several books dealing with these challenges. "Historical Sailing Ships -- Remote Controlled" and "An Introduction to Radio Controlled Scale Sailing Models" are two I have.
The fore and aft location of the centre of lateral resistance can reasonably guess-timated by cutting a thin plywood profile of all the underwater portions of the hull and then balancing on a thin dowel. If you have highly detailed copies of the full size plans, you may find a circle divided into quadrants located somewhere in the keel. This is the naval architect's calculation. If you balance your area adds in front of and behind this location, you won't disturb things -- same thing for sail area.
There may also be a similar circle marking for the centre of mass (and buoyancy).
If you have the centre of mass marking, keeping you fore and aft ballast adds/removals will keep you in the ball park but battery, receiver, and servo locations (and lack of engines) and difference between building methods can disturb things. Keeping your internal design options flexible to allow some shifting around after pond testing is desirable (but if strict scale modeling is desired, location choices may be limited.
If you're not a racer, adding some area to the rudder will allow you to make up for small errors in the relationship between centre of lateral resistance and centre of effort (sail area). You'll go a little slower.
With respect to early discussion on frame spacing. Feel free to use only every 5th or so frames. As long as spacing is around 10 cms or less, you should have enough strength. Add some in between frames if needed for strength in key areas (e.g., if your mast step is going to be on deck instead of the keel) or you have a complex shape transition area (not likely looking at your design).