I cannot find these engineered wood pieces for the 9'*5" gap in my living room that I need to fill. Since the ones out of the box would anyways be much lighter than what I have on the floor already, can I substitute with a different type of wooden floor planks?
If it's a floating floor, your best bet is to take it apart, then put it back together mixing in the new boards so they're not all in one spot.
That said, are you sure the new boards won't match the installed ones? I'm assuming this might be on the edge of a room, so there may not be all that much fading to begin with.
As for using something else, you certainly could, but you'd likely have to go with at least the same manufacturer to ensure the joints match. And I doubt you'd get a better match than the ones you already have in the box anyways.
Brazilian Cherry darkens with light but it only darkens to a certain point. Put the stuff you have in stock on the floor. Live with the color variance for a few months until it blends. If it really bothers you a few months later because it hasn't changed then cover up surrounding area and put a grow lamp on it until you are happy.