You can use the 75-inch gradient light strip on your 86-inch TV without any problems (I also do this). You just have to make sure that the lights are not too close to the edge of the screen or leave a fairly large gap. But don't worry, the dividers shine so incredibly bright that it still works almost perfectly.

No, the Gradient Lightstrips are not extendable. This is the extension to the basic set Ambiance 16467480.