The "Rutherford experiment" was actually the "alpha scattering experiments" carried out by Hans Geiger and Ernest Marsden. They used a number of different metal foils and of different thicknesses before passing their results on to Rutherford for later analysis. Gold is useful because it can be formed into extremely thin sheets and gold has a nucleus with a large electric charge, which is able to substantially deflect alpha particles. With fewer protons and less electric charge in the nucleus, a thicker piece of aluminum foil would not have shown the unusual scattering of alpha particles as well.
It is important to understand that it isn't the size of the nucleus that is as important as the electric charge of the nucleus. Yes, a nucleus of aluminum is smaller than a nucleus of gold, but gold has a charge of +79, while an aluminum nucleus is only +13. The large angle scattering of alpha particles is not due to a physical collision, but rather to electrostatic repulsion. The greater the charge on the nucleus, the greater the repulsion, and the greater the scattering angle.
