Don't worry too much about varying differences in wood quality (at this level anyway), just use the lighter one on areas of less stress, if these pieces are used in the wing (spars), use the stiffer ones starting from the center of the wing, lighter ones goes towards the tip (less stress), the piece that broke, don't loose any sleep over that either, you can just use some glue to weld the joint, use some pins to hold the parts while it dries, or just use cyano, after the part is glued where it is supposed to go, you can just reinforce the joint with some scrap sheeting material (3/32 is fine) on two opposing sides, that should be plenty.
When building wings, just be careful to try and even out the quality of the wood used on each wing, this is to avoid having one wing heavier than the other, which in turn can change how your airplane will behave in flight