Xeretai Niko,
Kitaxo kai yelao!
Both for the rest of us on the forum, and because my Greek is household rather than technical: I will switch to English.
There are way too many design issues for us to talk about in just one forum, and when I can't really answer a question completely, I choose to keep my mouth (keyboard?) shut.
But you called me out, so here I am. I promise not to give you the answer to "Life, the Universe, and Everything" in fifty words or less.

Pattern design is a tricky business with only a few good rules, and lots of conflicting theories.
On top of that, the behavior of airplanes is very non-linear: even a amall aesthetic change to an existing airplane will sometimes produce large changes in how it flies.
and other times, dramatic changes accomplish nothing.
At best, the experienced designers have collected more "rules of thumb" about what not to do.
After that, we are all slaves to fashion, and will spend lots of time trying to trim out the designs we drew just because they were pretty.

There are Pattern designs with the stab very close to inline with the wing,
and those where the stab is much higher and "out of the dirty air" as the proponents of this school of thought say.
There are examples of both kinds of design that fly very very well, and there really is more than one way to solve the design problem.
If you are eager to learn it all, then you are going to have to build a few airplanes, or at least one with all plug-in feathers, and a simple fuselage with many wing and stab tube holes!
In the meanwhile, you might choose to pay a flattering compliment to a proven designer, and steal ... okay, borrow the "numbers" of a successful design.
The wings, tail, and thrust-line placement of a proven design can provide a skeleton around which you can draw something you like.
Then start making small changes and see what happens.
Here's an interesting one for you: the Oxalys is obviously a very successful design, but all of the ones I have seen needed lots of rudder into UP elevator mix in order to fly properly. Why? It turns out that the canaliser (or "flow straightener in English) greatly helps this problem on some designs, and does nothing for others.
Why not start out with the Oxalys skeleton and see what changes with a different fuselage shape.
My pet theory is that the Oxalys suffers from a strange airflow when it is yawed: the air separates around the tall canopy and then turns toward the belly aft of the canopy.
In my opinion, this is the cause of the strong pull to the wheels with rudder, and a fuselage shaped more like a fish will not suffer this problem. So will one with a spine that stops the cross-flow.

I have designed more than just a few really good airplanes over the years, and still have more questions that answers !
Later I will attempt to answer your question about taper ratios.

Yia,
Dean Pappas