That's a really strong Ryobi. Is it modified? Are you sure your tach is correct? My 42cc Poulan turns an 18 x 8 at about those rpms...

If your tach is correct, then personally, I would prop it a little heavier to get closer to 7500 static as you say. The prop unloads in the air, and the consequent rpms will be high for a Ryobi.

As far as which prop, that depends on both you, and the airplane. A big slow airplane would like the 20 x 6, as would an airplane used for some 3D flying. A faster, sleeker airplane used more for traditional aerobatics would probably do better with the 18 x 10. You prop the engine according to the airplane, and also according to it's intended use.

Have fun,
AV8TOR