I found that my Poulan does not have enough torque to unload 18x6 on the ground. My test prop MA 16x8 Classic has more rpm (7400) than NX 18x6 (6200). The best prop for maximum performance for my Poulan 25cc was APC 17x6.

As for quarter scale Piper, both combinations should work well. It is a big high-lift wing and does not need much speed to fly. With 18x6 I guess you will not be able to fly faster than 40 mph (about 1.5 of stall speed) - not much reserve for wind gusts. With 16x8 you should still have enough thrust (over 9 lb of static thrust) to take-off and fly the plane with reasonable performance and you may actually be able to fly faster (about 2 times of stall speed). You may also consider 17x6 as alternative to 18x6 (it should give better transition).

Either way I would expect such combo to fly like a real thing - smooth and slow.

Actually I'm building CoroCap 96 inch span as a "test plane" for my engine. I'm going to use MA 16x8 Classic for the look and speed (coroplast is a little heavier than balsa).