The license requirement is for the entire experimental class, not the just Mosquito specifically. In the experimental class ONLY you may fly any category of experimental aircraft with any private pilot license. If you are licensed for balloon you could fly fixed wing or rotor wing as well. If you are licensed for heli you could fly fixed and balloon etc. etc. BUT this only applies as long as you are the only one on board (absolutely no passengers at any time in an aircraft category you are not rated in) and that aircraft can not have more than two seats - you could not fly a 4 place fixed wing on a heli license but you could fly a two place. All of that said you still need to learn to fly the category of aircraft you will be operating and since add-ons to your primary rating are 15 hours dual/15 solo by the time you learn the new aircraft type you are almost at your license anyway.