To pick the best prop, the best thing is to find an engine test report and see where the peak hp line on the graph is. Pick a suitable prop to be just slightly slower than that rpm, as it will unload a bit in the air. It will likely be around 13,000 rpm in something like an FP. Running at full rpm will mot hurt most two strokes, they like it, unless they are too lean. Glad you got it going well. Just so you know, after blowing a few plugs, it is a good idea to check for white pieces of the element that stick to the piston top. This should be removed as it causes plugs to blow even further, much like bearings and plating flakes going through an engine. Your problem is unrelated to this however, and your plugs are likely just fine.