Scratcher, I like Ben’s answer. I am also a career aero engineer (retired) and with the possible exception of Burt Rutan few full scale designers start from scratch. Aircraft design is highly empirical and is a process of evolution. A good bit of my experience is in engineering flight test, and I can confirm that surprises frequently arise there, even with the best of engineering design. I will try to address your last two questions.

The main advantage of locating the horizontal stabilizer a little above the wing, in an aircraft of conventional configuration is to remove it somewhat from the influence of the downwash from the wing. This generally provides a more linear stability curve than is the case where it is in line with the wing. The “T” tail has several other effects. By acting as a tip plate it increases the effect of the fin/rudder allowing a smaller area to be used. Some of this benefit is negated by the weight of additional structure required to support the stabilizer loads. For model aircraft, these effects are not particularly significant. In fact since many models are designed for aerobatics, the preferred position is in line with the wing which itself usually has a symmetrical airfoil. The reason for this is so that the aircraft will perform upside down the same as right side up.

All of the “rag wing” pipers (cubs, tripacers, colts, etc.) as well as many other of the fabric covered aircraft of that era, all have tails with flat surface airfoils. As far as R/C models there is no size that requires an airfoil shape. It’s really a matter of drag. An airfoil shape has a little less drag than a flat surface moving edgeways through the air. However I doubt if you could ever tell the difference in a model since they are so overpowered compared to full scale machines. In addition to the slightly lower drag, there could be a little difference in the control effectiveness at large deflection angles, but for model design it shouldn’t be significant. A flat surface is just simpler to build.