RE: O.S. Gemini 120 twin
22,000 RPM is what you would see when using a ducted fan system. To get that RPM with a propeller would require an astonishingly-small one. Really, you should shoot for 15K or so, but you will likely never be satisfied with the performance, because if you get a prop that will give you a reasonable top end, you'll have a lousy idle. Wind up with a good idle, and you'll likely have a lousy throttle transition and maybe a lousy top end. With a ducted fan, the idle was 5,000 RPM or so...not something you want with a propeller. You can play with it and see if you can get it to work, but we'd not have confidence that it will suit.