If you do the math with a calculator you'll, in my opinion, never understand modulus. Use paper and pencil for some small numbers and you'll see how modulus works.

I believe to randomize 8 LEDs all you need to do is change the 6 to an 8. Like
Random myword
mybyte=(myword//8)

Depending on how you use mybyte depends on whether or not you need to +1 after myword//8.

I'm sure someone will chime in and say I've messed up but there you have it.