The simple belt and braces approach is to work out the centre of balance of the under water profile including rudder and balance this with the total centre of effort of the sails.
Trace the under water profile including rudder from the plans and cut it out of stiff cardboard. You need to balance this on a knife edge by laying it across the edge so that the knife edge is oriented 90 degrees to the water line.
With your hull shape it should balance roughly half way between bow and stern at the water line. Don't let the profile sag over the knife edge, fold concertina type creases forward and aft to stiffen it up if needed. Balance it horizontally and you have found the centre of effort of your underwater profile.
Next choose your sail plan and take moments about a convenient point (sail area times distance from chosen pivot point), say the end of the bowsprit. But first you need to find the centre of each sail as this is coincidered to be where the driving force of that sail acts from. So the moment of that sail is its area acting from its centre of effort times the distance from the end of the bowsprit.
Do this with all sails and the sum resultant is a distance from a point dropped vertically to the waterline at the end of the bowsprit. The resultant will be a distance from this point aft along the water line.
The centre of effort of the under water profile has to balance with the centre of effort of your sails and for your boat I would guess they should be close together with the sail area in front of the under water profile by a small distance.
This is called the 'lead', it is the distance between the plotted centre of under water profile and the combined centre of effort of the sails. It is usually expressed as a percentage of the water line length. With your boat and a chosen rig of gaff mizzen, gaff main with topsail, foresail and jib on a bowsprit, the sail C of E will be just in front of the under water C of E by 0.7 to 0.11 of the water line length.
Play about with sail areas at the drawing stage until you have this small lead of sail area over underwater profile.
If the sail C of E is too far forward the boat will turn away from the wind with the rudder central (lee helm) and if the Cof E of the sails is too far back the boat will constantly want to turn into the wind (weather helm) Slight weather helm is good, any lee helm is bad i.e. the need to put the tiller to leeward to keep the boat on the wind.
By altering the underwater profile of the completed boat, by changing your detachable fin profile fore and aft you can correct the above problems by experimentation - this is the fun of playing with sailing models
Or you could shorten or lengthen the bowsprit or change sail areas.
Changing sail plans on a completed boat takes more effort than changing the under water profile by means of altering the fin in my humble opinion.
The point I'm trying to make is that it isn't a total disaster if you get it a little wrong and the model can be modified by sailing the thing and see what happens and then take steps to fix it.
I did all the above while designing my model sailing canoe and found that I had goofed a little
To correct things I needed to make the mizzen sail area smaller to get the boat to balance (in my case it was easier to do this than move the dagger board and its case) I could have added a short bowsprit and a jib and this would have had the same effect.
Don't let your model building stall because your stuck for a sail plan and worrying if it all will work out! Choose a sail plan that you like and do the above so your in the right ball park and get the thing sailing!
There two books I would recommend that describes things far better than I if you want to go into things at length, these are 'Yacht Designing and Planning' by Howard Chapelle and 'Elements of Yacht Design' by Norman N. Skene
Both are easy to read and with a little head scratching easy to understand.