Try the 14.5 x 11. If anything it will load up the engine too much and you will have learned something. It probably will load it down well below 8000RPM. A two blade 14.5x11 would turn about 8500-9000 RPM on the 1.20. I think you will start running into trouble with the engine-prop combo more than the prop-airframe setup. If you intend to keep the same flight speed it makes little sense to reduce pitch when increasing the number of blades. Yes it does reduce the load, but it will also reduce speed.