I don't have experience with this exact plane, but I helped a guy with a Nitromodels Bobcat 50 once. He had the similar rotation problem that you describe. The problem was that the main gear were too far behind the C.G. for the plane to rotate and achieve the necessary angle of attack needed to lift off. There are two ways to correct this angle of attack problem. You can remount the main gear so that they are about an inch behind the C.G. or you can lengthen the nose gear. Lengthening the nose gear is probably easiest and allows the "too far aft" main gear to protect the prop better from ground contact.

The Bobcat 50 was also this guy's first pusher and he made a number of "rookie" mistakes setting it up. He had a pusher prop mounted but he put the prop on backwards by mounting it with the sharp, trailing edge of the prop towards the front of the plane. Pushers props are just like tractor props in that they need to bite into oncoming air with the rounder leading edge for best efficiency. This is a common mistake for first-time pusher owners. Another common mistake that he made was to mount the tank with the clunk dangling forward to the nose of the plane. Again, pushers are no different than tractors here in that they need their tanks set up with the clunks dangling to the tail of the plane so that a sustained climb with less than half a tank of fuel will keep the clunk submerged. If you haven't already done so, you might try checking out the threads on the Bobcat 50 for more tips.

Also, I have an O.S. 25fp on a pusher plane called the Sig Tristar. This engine seems to be really happy turning a Tornado 8/6 pusher. I get about 14k rpm with it on the ground. I think you might be over-propping a bit. Shorter props make for better ground clearance too, so I think it will probably be a win/win situation for you to try an 8/6.