RE: which props are recommended
Control line planes in the .15 to .35 size range need a decent amount of speed to be maintained to keep good line tension and control authority. There is no point in getting started in C/L with an engine prop combo that compromises the planes' ability to maintain line tension. If you want to start out slow, use the .15 with either a 8x3 or 8x4 Master Airscrew prop on a plane with a fat airfoil. Save the 3 blade props for later when you become an accomplished Ginsu Masta of the stunt pattern and your plane and skill level is finely tuned enough to nurse a slow moving .40-.60 sized ship through the stunt pattern. If you insist on a 3 blader for the .15, try to find a 7x3. Most .21s that I know of are high revvin' engines that need a easy prop to turn, most will have a very narrow range of props that they will be happy with. you might find that a 7x3 or 7x4 three blader is all a .21 will turn with any sort of willingness. The AMA might still have a chart that shows recommended engine / prop / applications, though last time I saw that chart, it seemed like it needed some updating.