Is all "cotton velour" 80% cotton/20% poly? Can I use this to make diapers?

I've never sewn with cotton velour, but I"m pretty sure that's the norm. The poly is in the base of the fabric and helps to make it more durable.

Not 100% sure, but pretty sure.

Look up some cotton velour dipes and see if they list their content.

I've found cotton terry velour at Hancocks that was 100% cotton. It's terry textured on one side, velour on the other, and plush as can be. I actually made mama cloth out of the scraps when I was done making NB fitteds, it was so nice.