Balsa actually has higher strength to weight ratios at the higher densities. Nevertheless, for small glider wings and stabs, you probably want to use thicker sections of low density balsa, usually covered with tissue for better bending strength and stiffness. You should try to use the densities and materials called for in plans. Balsa is an organic material and properties vary. Some people suggest that if you don't have the right wood, your just wasting your effort. However, I suggest you use what you can get, get a feel for the wood through experience, and perhaps use thinner sections of balsa to compensate for higher density, etc.